Streamflow and Water Quality:
What does the science show about clearcutting in western Oregon
George G. Ice
Clearcutting has been vilified because of potential effects on water quality and runoff. A Native Forest Network Campaign web site states that clearcutting “...exposes the soil to erosion, water storage capacity is lost, ... streams or rivers are loaded with sediment, killing fish populations (http://www.nativeforest.org/campaigns/neforest/clearcut.html). Findings in the OLIFE Initiative were that clearcut logging “...substantially increases the likelihood of large landslides and severe flooding...” and has resulted in “...serious degradation of Oregon’s surface and ground water supplies by increasing sedimentation and turbidity, adversely altering the chemical composition of such waters...” (http://www.tuwdesigns.com/olife/ifull.html). Even a recent National Geographic article depicts logging as causing “...serious problems for streams as dirt pours off clear-cut slopes...” (Mitchell and Essick 1996). Fortunately, western Oregon benefits from a number of watershed studies which have included tests of clearcutting impacts on water resources. This paper reviews relevant research from western Oregon and elsewhere to show how clearcutting affects streamflow and water quality. Topics covered will include the affects of clearcutting on runoff, especially peak flows; sediment; nitrogen/phosphorus; temperature; and dissolved oxygen.
How Clearcutting Changes Watersheds
Clearcutting is an even-aged regeneration method defined as “the cutting of essentially all trees, producing a fully exposed microclimate for development of a new age class” (Helms 1998). Key watershed changes following clearcutting include: removal of the overstory vegetation which reduces interception, evapotranspiration, and nutrient uptake; increased variations in temperature and increases in wind velocity at the forest floor; and changes in wind and air turbulence patterns at the clearcut/forest interface. Additional impacts from yarding that necessarily accompanies clearcutting, to greater or lesser degrees, includes ground disturbance and soil compaction. Chemical and mechanical site preparation and prescribed burning are also commonly associated with the re-establishment of stands, resulting in additional changes to understory vegetation and the forest floor and soil. In separating out the impacts of clearcutting we need to consider both what impacts can occur and the practices that can minimize undesired hydrologic changes. Also, unlike other land-use activities, forests that are harvested re-grow and changes in the watershed are reversed over time (Figure 1). Modified hydrologic conditions influence only a fraction of a large forest watershed at any time.
Figure 1. Needle Branch After Clearcutting, Prescribed Burning, and Debris
Removal in 1966 and Twenty Years Later
Runoff Response to Clearcutting
Any consideration of water quality impacts must start with an understanding of how water is routed to streams. Hydrologic responses to clearcutting can be modeled using a water balance based on the hydrologic cycle to look at how precipitation is routed out of the watershed (Figure 2). Unlike the eastern US, where precipitation may be evenly spread over the year and thunderstorms and hurricanes may create high intensity events, in western Oregon precipitation patterns are dominated by relatively low intensity, long-duration storms, with about three-quarters of the precipitation occurring from October through March (Harr 1976). Even extreme events like the February and November 1996 floods resulted from precipitation intensities which would occur much more commonly in other regions. The November storm produced one-day precipitation records for some rainfall stations, with totals of 2 to nearly 7 inches (Robison et al. 1999). The U.S. Weather Bureau has estimated 6-hr probable maximum precipitation (PMP) for 10-mi2 watersheds of around 13 inches for much of western Oregon (Ward and Elliot 1995). The 6-hr, 10-mi2 PMP for most of the southeast is between 30 and 32 inches. While rain dominates the Coast Range and Willamette Valley areas, transient and persistent snow pack can be important in the higher elevations of the Cascades.
Harr (1976) found that “in undisturbed forest soils in western Oregon, infiltration capacities far exceed the maximum rates of rainfall so that all water enters the soil.” Yet even undisturbed forest watersheds have streamflow. This results from rapid translatory flow, downslope drainage and return flow (either as quick subsurface stormflow or delayed base flow). In some cases as much of 80 percent of rainfall will move out of a small forest watershed in the first 24-hours. Drainage of water from upslope creates higher water conditions on the lower slopes. Computer models, like the Distributed Hydrology-Soil-Vegetation Model (DHSVM) (Storck et al. 1997), which computes water balances on individual pixels and then route runoff (surface and subsurface) between pixels, may eventually improve our understanding of the spatial influence of forest management activities. Pathways and source areas for runoff can affect water quality
Figure 2. A Simplified Hydrologic Cycle for a Forest Watershed
response to clearcutting. For example, water coming from unsaturated subsurface flow may have increased nitrate loads, while saturated soils are locations for nitrogen loss due to anaerobic denitrification. Similarly, sites of return flow or surface runoff are areas of increased hazard for soil erosion, especially where disturbance has resulted in easily detachable particles.
Potential changes in peak flows are among the most controversial issues for forestry in general and clearcutting in particular. Much of the controversy arises around the definition of peak flow that is used. Often the peak is equated with flood flow, which is some stage that goes over the streambank and causes damage to adjacent property. Peaks used to assess forest management impacts may be events that occur every year or two or even events that occur several times each year. Of special interest to hydrologists are annual peaks (maximum flow that occurs in a single hydrologic year–October through September), two-year recurrence events (flow occurring about once every two years and often equated as the bankfull discharge), five-year recurrence events (flow which may cause significant modification to channel morphology), and flow of record (highest discharge ever recorded).
There are numerous mechanisms by which clearcutting can potentially increase peak flows including:
· reduced interception caused by removal of the forest canopy during harvesting
· reduction in evapotranspiration caused by loss of vegetation, especially deep rooted trees
· modified snow accumulation and melt rates
· reduced infiltration due to frozen soil
Associated activities of yarding and site preparation can:
· reduce infiltrations rates due to soil compaction and disturbance, and development of water repellent soils
· increase efficiency for the runoff network by removal of wood in channels or the construction of roads with ditches
While these mechanisms can potentially increase peak flows, there are also compensating mechanisms which occur with each. Reduced canopy interception may be partially compensated for by increased interception from slash or the shrub canopy and eventually by re-growth of the forest. Reduced evapotranspiration is partially compensated for by an increase in evaporation. Despite these compensating mechanisms, research in western Oregon shows that some peak flows are increased from management associated with clearcutting.
The removal of trees reduces interception, both of rain and snow, and evapotranspiration. In the Northwest, major floods occur during the “dormant” fall and winter seasons. Interception in particular has come under additional scrutiny over the last year. Reid (1998), based on interception data from New Zealand, has estimated that a mature redwood forest can intercept 20 to 30 % of precipitation during a major storm. This appears to greatly over-estimate the influence of interception for major storms in the Northwest, especially when antecedent moisture is high, but the combination of reduced interception and evapotranspiration following harvesting does results in greater soil moisture for clearcut areas, particularly in the early fall. Watershed studies in the Oregon Coast Range (Harris 1977) and Oregon Cascades (Rothacher 1971) concluded that clearcutting affected the size of early small peaks, when antecedent moisture levels were low but did not significantly affect large peaks.
In recent years some of these early conclusions have been challenged. A study in Caspar Creek in coastal California has reported increases in biannual (about twice a year) peak flows of 35 % for completely clearcut subbasins (Ziemer 1998). However, changes in peak flows were difficult to detect downstream and any changes in peak flows were judged to be “relatively benign.” Similarly, reassessment by Jones and Grant (1996) of long-term runoff records for a clearcut watershed in the H.J. Andrews Experimental Watershed found that “...the entire population of peak discharges is shifted upward by clearcutting and roads, we see no reason to expect the biggest storms to behave differently from the rest of the population.” A re-evaluation of this data by Thomas and Megahan (1998) found a decreasing response to clearcutting as both the magnitude of the event and the time since harvest increases (Figure 3).
Figure 3. Analysis of Peak Flow Response for Watershed 1 (Clearcut) to
Flow in a Control Basin and Time Since Harvest (Figure
provided by Thomas and Megahan)
This debate re-enforces the importance of clearly defining the peak flow change of concern. It also points out that while changes in small, early peaks are real, the impacts are generally subtle and transient compared to other land-uses or to variations in weather. As a comparison, urban development on a watershed, with increased impervious surfaces and drainage systems, can dramatically and permanently increase peak flows. An analysis of the 30 mi2 North Creek Watershed in Washington found that the 100-year flood flow had increased from 680 to 1440 cfs due to urbanization. Similar analysis for clearcutting might be expected to yield no more than a 5 to 10% increase. This is within the range of flow measurement error.
Peak flows, and especially floods, result from extreme precipitation events and antecedent conditions that are conducive to producing runoff. Much more “noise” in peak flows results from difference in precipitation patterns than from the impacts overlain by forest management. An example of this is an analysis of peak flows for Forest Creek in the California Sierra Nevada Mountains. An earlier analysis of Forest Creek found a statistically significant relationship between the residual of excess flow over expected flow with time, causing some to concluded that more roads and clearcutting in the watershed were causing peak flows to increase. Munn (1993) re-analyzed this relationship to consider antecedent conditions using pre-storm flow as well as temperature. Temperature determines whether precipitation is in the form of rain or snow and is a factor in snow melt rates. When these factors were included in the analysis, the excess flow over expected peaks became non-significant over time. This assessment is supported by publications showing how recent climate changes, especially temperature, have influenced peak flows in the Sierra Nevada Mountains (Pupacko 1993).
Impressions about floods and peak flows are generally based on what has recently been experienced. The floods of February and November 1996 have reset much of the thinking about watersheds and their responses to management. Yet even these event are dwarfed by past floods, at least at the regional scale. Miller (1999) recently summarized the flood of 1861. The February 1996 flood had a flow of 127,000 cfs in Albany. This is dwarfed by the 1964 Christmas floods with a peak flow of 183,000 cfs in Albany. But both these floods occurred after the construction of flood control reservoirs in the upper Willamette and McKenzie River systems. The 1861 flood is estimated to have had a flow past Albany of 340,000 cfs. So even before management had any significant influence on forests, floods occurred.
Clearcutting tends to increase the amount of snow accumulation. This results from reduced interception loss (snow in canopy more available for sublimation) and can also involve redistribution of snow into openings. For transient and persistent snow packs, the combination of increased snow accumulation, elevated winds causing accelerated snowpack melt, and increased net precipitation can potentially cause increases in peak runoff. However, deeper snowpacks can sometimes absorb rain and snow melt and may not contribute runoff during a flood event. There have been a mixture of results from watershed studies of the effects of clearcutting on rain-on-snow peak flows. Some of the runoff peaks occurring in the H.J. Andrews Experimental Watershed are probably rain-on-snow events.
Changes resulting from management activities are usually evaluated against control watersheds which have not had recent management activities. This approach fails to consider the role of natural disturbances such as major windthrow events like the 1962 Columbus Day Storm or, especially, wildfire. Some of the most spectacular changes in watershed discharge have resulted from extreme wildfires where organic matter in the forest floor burned and water-repellent soils formed. A severe wildfire in the Boise National Forest, which created extensive water-repellent soils, is believed to have caused a 1- to 5-year recurrence thunderstorm to become a thousand year flood event.
Some changes resulting from clearcutting, such as reduced interception and evapotranspiration, cannot be avoided. Other changes, such as soil disturbance and compaction during yarding of logs, can be minimized by the use of full or partial suspension cable systems, designated skid trails, or other management measures.
Grant, Megahan, and Thomas (1999) recently reviewed their recent papers and came to a series of talking points provided in the attached Appendix. Ziemer (1998) concludes that “...the greatest effect of logging on streamflow peaks is to increase the size of the smallest peaks occurring during the driest antecedent conditions, with that effect declining as storm size and watershed wetness increase. Further, peaks in the smallest drainages tend to have greater response to logging than in larger watersheds.” The overwhelming factors determining floods are precipitation and antecedent soil moisture and snowpack conditions. As stated by Dr. Seuss:
The storm starts when the drops start dropping,
When the drops stop dropping, the storm starts stopping.
Water Yield and Low Flow
Water yield and low flows can also be influenced by clearcutting. Initial effects are an increase in water yield and low flows as evapotranpiration and interception are reduced. These benefits will decrease over time. Unfortunately, most of the water yield increases occur in the fall and winter when increased water is least needed. In some long-term studies, after initial water yield and low flow increases, it appears that summer low flows may actually drop below pre-treatment levels. Some possible mechanisms include reduced stomatal resistance by species change, increased evapotranpiration by riparian phreatophytes (no longer shaded by upslope vegetation), and reduced fog drip. Changes to peak flows, low flows, and water yields from clearcutting and selective cutting are generally related to the proportion of the vegetation removed, although there is also some affect of location where the vegetation is removed. Near stream vegetation removal will generally result in more runoff than removal of vegetation on the upper slopes. Because only a small percent of any large watershed will be covered by recent clearcuts, any benefits or possible negative effects on water yield and low flows will generally be diluted to below detection limits.
Sediment and Clearcutting
Accelerated delivery of sediment to streams is considered by many to be the most significant environmental impact resulting from forest operations. Increases in suspended sediment and turbidity can cause fish to avoid tributary streams, delay their migration, reduce feeding, show increased stress, and, at very high concentrations, die. Probably more important than these direct impacts are effects on habitat, including sediment deposition causing loss of pools, widening of the stream, and filling and burying of gravels.
Sedimentation involves four basic processes, whether it is sheet or rill erosion, channel erosion, or mass wasting. Material must be detached, entrained, transported, and deposited. Practices which can minimize these processes can minimize sedimentation. Forest management can affect sediment loads either by increasing the material available for transport (for example by increasing detachment as a result of increased bare mineral soil or concentrating flow onto an erosive slope) or by increasing the runoff available to transport material (increased sediment carrying capacity).
Some potential mechanisms for increased sediment from clearcutting, yarding, and site preparation include:
· disturbing or exposing soil susceptible to detachment during yarding, roading, or prescribed burning
· increasing surface runoff due to reduced infiltration caused by compaction or formation of non-wetable soils
· causing erosion from concentrated flow from roads, landings, and skid trails
· increasing mass wasting due to site disturbance (log gouging), windthrow of residual trees (buffer windthrow), root strength reductions, and changes in site hydrology (reduced ET, concentrated flow)
· reducing in channel flow resistance and sediment trapping by removing stable wood
· loading channels with mobile debris
In the Alsea Watershed Study, suspended sediment clearly increased following harvesting and site preparation for the completely clearcut watershed. The first year after clearcutting and site preparation Needle Branch sediment output was about six times the 7-years pre-treatment average while the control watershed had about average sediment runoff. These increases dropped rapidly, and the 8-year post-treatment sediment discharge averaged about three times the pre-treatment discharge for Needle Branch (See Figure 4). Most of this increase was probably not a result of clearcutting but of the post-harvest treatment of Needle Branch, where the watershed was prescribed burned and debris was cleaned out of the stream by a tractor moving up the middle of the channel. Deer Creek experienced more modest increases in suspended sediment. These were mainly associated with landslides from sidecast road failures in the watershed. Hewlett (1979) showed a similar pattern for sediment after treatment for a watershed in the Piedmont of Georgia, and how these increases in annual sediment yields expressed themselves over a full rotation. For Needle Branch this is about a 30 % increase in sediment over a 50-year rotation.
Figure 4. Normalized Suspended Sediment Response for
Needle Branch Following Clearcutting
The variability of watershed response is demonstrated by comparing sediment yields (tons/mi2 ) from the control (Flynn Creek) and the treated (Needle Branch) streams. At no time, even immediately after harvesting and site preparation, did Needle Branch experience unit area sediment loads equal to the maximum sediment yields measured in the control stream. Also, the estimated long-term suspended sediment yield (tons/mi2 ) for the control watershed is higher than the estimated long-term suspended sediment yields for Needle Branch.
Sediment yields from experiments on the H.J. Andrews Experimental Watershed are more difficult to interpret because of landslides. For the watershed that was completely clearcut, there was almost no response during the first several years of harvesting, but a large increase occurred following slash burning through the stream channel (Larson and Sidle 1981). This apparently released stored sediment. Landslides in the watershed also contributed to some increases compared to the control watersheds.
In both these cases it was probably the post-harvesting treatments, especially burning and removing wood in the stream channels, which resulted in much of the increase in sediment observed. Beschta (1979) recorded the erosion of sediment deposits following the removal of large wood for a creek near Alsea, Oregon. Megahan and Nowlin (1976) measured sediment stored behind obstructions for small streams in Idaho and estimated that the stored sediment represented about 10 years of sediment yield. Like peak flows, assessments for affects on sediment are usually made for undisturbed watersheds without considering the role of natural disturbances in creating large pulses of sediment. Somewhat controversial work measuring cosmogenic isotopes in sediments indicates that current sediment exports from watersheds are less than the long-term rates of the last 10,000 years, possibly due to reduction in forest wildfires (Kirchner et al. 1998).
Some management practices commonly used to minimize sediment impacts from clearcutting and yarding activities include: controls on yarding such as full or partial suspension, uphill cable yarding; water bars on skid trails to avoid concentrated flow and to disperse sediment into the forest; hydromulching and other ground covers to protect bare soil; and management restrictions near channels to avoid removing or disturbing wood and sediment stored in the channel. For example, Froehlich, Aulerich, and Curtis (1981) found that designated skid trails could reduce soil disturbance by 45 to 80% compared to conventionally thinned stands.
Clearcutting has also been linked to increased landslides. While this topic is discussed elsewhere in this conference, landslides can have important impacts on streams and water quality as discussed above for the H.J. Andrews watersheds. Those interested in clearcutting and landslides should read the Oregon Department of Forestry report, Storm Impacts and Landslides of 1996 (Robison et al. 1999). Clearcutting may be more important in determining the timing of landslides than in causing landslides.
Two dated but useful references on forest erosion in the Northwest are the Erosion and Sedimentation Catalog of the Pacific Northwest (Larson and Sidle 1981) and the Catalog of Landslide Inventories for the Pacific Northwest (NCASI 1985).
Nutrient Response to Clearcutting
Nutrients are elements that are essential for plant and animal nutrition. While essential for growth at low concentrations, these elements can become damaging at high concentrations. The most common concern with nutrients is eutrophication of streams and lakes. Eutrophication is the process where a body of water becomes so rich in dissolved nutrients that lush growth (blooms) of algae and other objectionable plants results and seasonal or daily deficits in dissolved oxygen and fluctuations in pH occur. Other necessary environmental conditions must be present (light and temperature) for eutrophication to occur.
Nitrogen (N) can occur in many forms including ammonium ion (NH4+), ammonia (NH3), nitrite (NO2-), nitrate (NO3-), urea (CO(NH2)2), and various organic forms. For forest watersheds, the most important forms tend to be nitrate (because it is the most soluble and mobile form in oxygen-rich waters) and organic-N (bound to or part of sediment and litter) (Figure 5).
Clearcutting can disrupt plant uptake of N, increase mineralization of organic matter on the forest floor, and increase shallow subsurface flow to the stream. Prescribed burning
Figure 5. The Nitrogen Cycle
increases mineralization and volitalization of nitrogen. Application of herbicides retards plant re-growth and nitrogen uptake (and may contribute N, depending on the herbicide).
The classic study raising concerns about clearcutting and increases in stream nutrients is the Hubbard Brook Experiment in Maine (Likens et al. 1970). Dramatic increases in nitrate nitrogen and other nutrients resulted from harvesting on a small watershed. What is often forgotten is that this experiment was designed to observe the maximum nutrient output from a disturbed watershed. The watershed was inherently susceptible to nutrient runoff. It was clearcut, but the trees were left on the site. Repeated herbicide applications were made after harvesting to kill natural revegetation. Other studies in the area found that while nitrate concentations in streams adjacent to clearcuts increased immediately after harvesting, the young stands soon yielded less nitrate than mature stands. Use of streamside management areas also produced marked reductions in nitrate losses. Normal timber harvested appears to change the timing of nutrient outputs (elevated immediately after harvesting and depressed for rapidly growth stands) but not the total amount.
The concentration of N in streams can also be influenced by geology and plant communities. Foresters are familiar with the potential effects of N-fixing alder and Ceanothus on soil nitrogen and this is also found to contribute to stream nitrogen runoff. For example, in the original Alsea Watershed Study, the nitrate-N flux (discharge and concentration) initially increased by about 5 fold following clearcutting without a buffer in Needle Branch. Deer Creek, with patch cutting and buffer strips around the tributaries, showed no change in nitrate-N flux. Stednick and Kern (1992) have shown that the “control” watershed, Flynn Creek, had higher nitrate-nitrogen fluxes (kg/ha/yr) than either of the treated watersheds (Deer Creek and Needle Branch), both before and after treatment. This high flux appears to result from tributaries where the riparian plant communities are dominated by alder. Stednick and Kern (1992) also documented the recovery in N flux from Needle Branch. In the Mokelumne Watershed in California, accelerated forest harvesting associated with a change in ownership in the basin was accused of causing eutrophication in a downstream reservoir. A search for nitrogen sources found that low-elevation tributaries in high ammonia-bearing lithologies, contributing 5% of the total watershed discharge, were contributing about 90% of the nitrate load to the reservoir (Holloway et al. 1998).
In the H.J. Andrews Experimental Watershed, clearcutting of Watershed 1 caused little if any increase in nitrate-N loss, but there was a measurable increase following the prescribed burn (Figure 6). These concentrations do not pose a threat to aquatic or human health.
Phosphorus (P) also occurs in several forms including the dissolved forms of orthophosphates (also known as reactive phorphorus) and dissolved complex organics and particulate forms (organic and inorganic). Most monitoring is done for ortho-P and total P. Nitrogen and phosphorus ratios determine which of these elements is limiting to aquatic plant growth. A ratio of about 15:1 N to P (atomic) or 7:1 (mass) is commonly used to establish which is the limiting nutrient. If the mass ratio is greater than 7:1 then phosphorus is considered limiting. Phosphorus sources come from dry deposition (dust), wet deposition, and geologic weathering. Geology is a key factor in phosphorus loads from forests. Forests watersheds with
Figure 6. Nitrate-N Concentration in Streamflow Following Clearcutting
and Slash Burning for Watershed 1 in the H.J. Andrews (Fredriksen 1973)
more easily weather rock (i.e., sedimentary or volcanic tuff and breccia) have higher instream concentrations than watersheds with resistant rock (i.e., hard, intrusive igneous). Phosphorus is probably one of the least responsive water quality parameters to forest management. Several studies in the Northwest found no change in ortho-P, total dissolved-P, or total-P following harvesting. One study in the H.J. Andrews Experimental Watershed appears to have detected a very slight increase in ortho-P after prescribed burning. Total-P is strongly associated with suspended sediment. Practices which increase or control sediment will have similar effects on total-P. A study in Virginia assessing the effectiveness of the state BMPs found a 300% increase in total-P and a 560% increase in sediment-bound P following harvesting without BMPs, but no increase when BMPs were used (Mostaghimi et al. 1996).
Probably the most comprehensive review of P in forests is a report by Salminen and Beschta (1991). In a broad survey of streams, Omernik (1977) found that as the percent of a watershed in agriculture and urban land use increased, the mean total phosphorus concentration in runoff increased. Degahandt and Ice (1996) reported on Oregon Department of Forestry studies which found no apparent relationship between the amount of watershed recently harvested and summer phosphorus concentrations. Sullivan (1984) found phosphorus increased only where landslides added large amounts of sediment to a large river (Middle Santiam). Basnyat et al. (1999) found that management near a stream, rather than land use activities throughout a watershed, most determined stream nutrient concentrations. Streamside management zones and erosion control practices should minimize any negative changes in sediment-attached nutrients.
One of the paradoxes of nutrients is that fish productivity in streams may be limited by low nutrient concentrations of Northwest streams.
Temperature and Clearcutting
Water temperature is one of the most important factors affecting habitat quality for fish. Temperature influences fish in three important ways: by directly influencing physiological and biochemical rates; by affecting interspecies competition and fish pathogens; and by determining gas solubilities.
Stream temperature response to clearcutting is determined by changes to the stream energy balance. The temperature of streams is constantly moving toward an equilibrium with the temperature of the environment to which it is exposed. Some potential mechanisms for clearcutting to increase stream temperature include:
· changes in direct and indirect radiation as a result of shade removal
· increases in air temperature associated with removal of shade within the watershed and the stream-adjacent riparian area
· soil warming and its affects on shallow groundwater temperatures
· channel widening (resulting from channel disturbance or channel aggradation)
· changes in streamflow, particularly low flow
In the Alsea Watershed Study (Moring 1975), Needle Branch, which was nearly completely clearcut, showed large temperature changes. Prior to logging, Needle Branch had never experienced a stream temperature greater than 16.1°C (seven years of record) at the main gaging station. After harvesting, temperature increased to 22.8° C, and with removal of slash it increased to 26.1°C. Upper reaches experienced temperatures close to 30°C. Daily temperature fluctuations also increased. This response contrasts with what happened in Deer Creek where patch clearcuts and buffers were employed. In this case temperatures increased less than 2°C immediately after logging. The Alsea Watershed Study taught us that we can harvest trees in a watershed without major increases in stream temperature if riparian shade is maintained. That is not to say that changes in channel width or reduced summer streamflows will not create increased temperatures. But evidence at both the site and watershed scale indicates that only modest increases in stream temperature will occur if vegetation is maintained near the stream and severe channel changes are avoided.
It is important to recognize that temperature, like all water-quality parameters, is non-conservative. That means that increased loads upstream are not transported downstream without modification. Heat- and water-exchange mechanisms operate continuously to move the stream temperature from an elevated or lowered temperature toward the “temperature profile” of the basin. Zwienieck and Newton (1999) found that small increases in stream temperature through buffered clearcuts returned to the expected temperature within 150 meters downstream. Rate of recovery is a function of stream depth; the shallower the stream, the more rapid the recovery. Holaday (1992) analyzed stream temperature data for the Streamboat Creek Basin in the Oregon Cascades and found that improvements in stream temperature for upstream tributaries as a result of re-growth of riparian forests had no affect on temperature at the mouth of Steamboat Creek. Tributary inflows may serve as valuable thermal refuges. But don’t expect to change the stream temperature in major streams as a result of buffers on tributaries.
Stream size and function are important considerations for stream temperature changes due to clearcutting. Small, shallow, slow-moving streams with relative large surface areas are more susceptible to rapid heating. In the Alsea Watershed Study, it was the upper sections of Needle Branch with the least discharge that experienced the greatest increases in temperature. But, while small streams may be more susceptible to heating, they also recover from elevated temperatures more rapidly. Andrus and Froehlich (1991) studied recovery of riparian cover for small streams in coastal Oregon after disturbances. They concluded that “…within ten years, stream shading was similar to that provided by developed forests.”
The concentration of dissolved oxygen (DO) in forest streams is important for stream organisms, including fish. A number of early forest watershed studies demonstrated the potential for depressed dissolved oxygen when harvesting occurred near streams. During the Alsea Watershed Study (Moring 1975), surface and intragravel dissolved oxygen concentrations for Needle Branch were reduced to 2.5 and 1.3 mg/L, respectively, following harvesting on the watershed. These results led to forest practice rules which maintain shade over and keep organic debris out of streams.
Forest operations can modify dissolved oxygen by: (a) increasing temperature and consequently decreasing DO solubility; (b) introducing organic matter which is decomposed by microorganisms (with the consumption of DO); and (c) inhibiting the rate of oxygen input to the water (reaeration) by modifying streamflow.
A worst case scenario for DO impacts occurred in Needle Branch. In the spring of 1966 nearly the entire watershed was clearcut down to the stream edge. The stream was choked with sediment and organic debris and exposed to sunlight. In October the stream channel was cleaned of organic debris and the watershed was broadcast burned. None of the forest practice rules that are currently considered standard for fish-bearing streams, such as riparian protection zones, immediate stream cleaning, or precluding the use of fire in and around streams, were implemented. The surface DO concentrations were depressed in Needle Branch during the summer months in 1966 to values considered limiting to growth of juvenile coho (about 4 to 6 mg/L). In July, at times the surface DO levels actually went below lethal levels for juvenile coho (Moring 1975). The reduced DO observed in some reaches of Needle Branch during the summer of 1966 are attributed to the combination of increased stream temperature, reduced reaeration rates, and increased BOD from logging slash.
While this discussion focuses on surface DO, it is the intragravel DO which is critical during egg development. Oregon has water quality criteria for both surface and intragravel DO. Biochemical Oxygen Demand (BOD), caused by fresh slash, may be an even more important variable for intragravel DO because the reaeration process is not active in the streambed and mixing of reaerated water from the surface may be inhibited. Unfortunately, intragravel DO is more difficult to measure and characterize. The new Oregon criteria may be unrealistic. Skaugset (1980) found no significant difference in intragravel DO concentrations for undisturbed, partially harvested, and completely harvested watersheds. He did find that increases in inorganic fines were associated with depressed intragravel DO concentrations.
The Alsea Watershed Study taught forest hydrologists that DO concentrations could be severely depressed in small forest streams with massive accumulations of fresh slash and exposure of the stream to sunlight. Practices that avoid these conditions maintain DO concentrations in surface waters of turbulent forest streams near saturation. Intragravel DO can also be depressed with incorporation of fresh organic and inorganic material into the gravels and with heating of the stream. Management solutions are readily available to avoid these circumstances and have been incorporated into state forest practice rules.
Clearcutting increases peakflows, mainly through changes in soil moisture status or snow melt. Effects generally decrease with increases in the magnitude of the runoff event and the time since management. These effects are subtle compared to those from other land uses and from some natural disturbance events like severe wildfires. Changes in water yield and low flows are transient and diluted over watersheds. Watersheds clearcut as part of experimental studies have shown increases in suspended sediment yields. These increases are closely related to site preparation or yarding disturbance, and decrease with time since treatment. Special management protection near channels to avoid releasing stored sediments and upslope practices which minimize the creation of disturbed soils or placement of sediment in concentrated flows can greatly reduce sediment increases following clearcutting. Nitrate-nitrogen is more likely to respond to clearcutting than phosphorus. Again, impacts are subtle, short-lived, and minimized by careful riparian management. Temperature and dissolved oxygen response is also closely tied to treatment near water. Where shade is provided, only small changes in temperature can be expected, and these will recover rapidly downstream. Major reductions in dissolved oxygen are unlikely if fresh slash is kept out of streams.
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Harr, R.D. 1976. Hydrology of small forest streams in western Oregon. USDA Forest Service General Technical Report PNW-55.
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Talking Points on Forest Activities and Peak Streamflows from
Grant, Meghan and Thomas (1999)
1. Peak flow increases due to harvest activities are real (i.e., statistically significant) but may or may not be geomorphically and ecologically significant. At this point we just don’t know enough (but could make some educated guesses).
2. There is evidence that these changes occur in both small and large basins, but are probably more easily detected in the former than in the latter.
3. There is no evidence for a threshold below which changes do not occur – all the analyses suggest more or less linear trends with treatment intensity, at least initially. However, different hydrologic mechanisms may be responsible for the response to treatment over the range of flows.
4. There is fairly clear evidence of hydrologic recovery over timescales of 5 to 20 years, with diminishing peak flow response over time. There is some suggestion that basins with roads may take longer to recover than basins without (since the regrowth of vegetation does not apply to the roads themselves), but this has not been clearly established.
5. The greatest changes appear to be in the small to moderate (up to 2-year return period) flows. While these are not the biggest events that do the most work in mountain watersheds, they play some role in terms of transporting sediment, nutrients, wood, etc. The absolute effect of a change in peak flows for these fraction of flows is not well understood at this point.
6. The effects of harvest on peak flows vary by season and overall hydrologic regime – i.e., basins in hot, dry areas will have a more pronounced effect due to evapotranspiration mechanisms, while those in rain-on-snow dominated areas will be potentially sensitive to snowmelt driven effects. We have not yet clearly identified what the spatial extent of these different mechanisms might be, but could again offer some educated guesses.
7. The weight of hydrologic evidence is that the biggest floods are little affected by management, but the jury is still out – there just aren’t enough big floods in the record to get a clear picture of how management might be influencing them.
8. It does appear that the magnitude of peak flow changes is substantially less than the within-a-year and year-to-year variability in streamflows. In other words, the effect of management appears to increase peak flows of small to moderate size, but these changes are within the ‘normal’ range of variability of streamflows, at least for westside Cascade streams (all of the results so far apply only to this region). This is a principal reason why they are so hard to detect.
9. While we know quite a bit about streamflow and forest activities, we are not at the point of being able to state what the risk from these peak flow changes might be. My opinion is that the risk of large-scale system change due to peak flow increases of the type described here are slight – the system is just too resistant to change from fairly frequent small to modest events. But very little is known about what increasing the magnitude of small to moderate scale events on ecosystems might be – in general we know very little about the relation between the flow regime and ecosystem response – a hot area for current and future work. Managers (and society) are going to have to decide what level of risk they’re willing to live with, given what we already know.
10. Current procedures for evaluating hydrologic cumulative watershed effects do not really address the mechanisms involved (evapotranspiration, snowmelt, channel network extension by roads), although to the extent that any or all of these scale with total area harvest, then the ERA, ECA, or ARP type indices do provide some measure of the overall intensity of management activities. Spatial databases and GIS are making many of these indices obsolete, however. The next generation of cumulative effect analysis tools will advance our way of treating hydrologic effects. Key issues may turn on being able to identify the likely hydrologic function and effect of clearcuts, partial retention units, roads placed in different landscape positions with respect to each other, and the intrinsic ability of the landscape to process, store, and route water. The new generation of hydrologic models (i.e., PRMS, Wigmosta/Lettenamier) will play key roles in helping us work this out.
 Paper presented at: Clearcutting in Western Oregon: What Does the Science Show?, Oregon State University, Corvallis, OR. November 3, 1999.
 Principal Scientist, National Council of the Paper Industry for Air and Stream Improvement Inc., P.O. Box 458, Corvallis, OR 97339. Tel: 541-752-8801; Fax: 541-752-8806; email: Gice@wcrc-ncasi.