Streamflow and Water Quality:
What does the
science show about clearcutting in western Oregon[1]
George G. Ice[2]
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.
Peak flows
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.”
Dissolved Oxygen
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.
Conclusions
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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APPENDIX
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.