Streamflow and Fire Effects on Woody Debris Dynamics of Sierran Streams
Environmental Protection Agency/USDA-Forest Service,
USDA-Forest Service, Pacific Southwest Research Station, and
USDA-Forest Service, Tahoe National Forest
Little published inventory or trend information is available on woody debris in the Sierra Nevada. Periodically since 1991 we've tracked changes in the frequency, size, volume, stability and function of several thousand pieces of in-channel debris in 15 streams in the Tahoe and Sierra National Forests. One of the study watersheds was burned intensively in 1994. Ongoing monitoring of the stream in that watershed provides a database for before and after assessment of the effects of wildfire on debris dynamics. This article summarizes selected results of the study through 1997 and offers preliminary management recommendations on the treatment of in-channel debris after wildfire.
The basic approach used for all study reaches included tagging (with quarter-sized, numbered metal markers) debris pieces within the bankfull channel, measurement of piece diameter and length (excluding pieces <8 cm in diameter and <1 m long), classification of piece stability and geomorphic function, and assessment of sediment stored by the debris. To reduce variability on six of the streams we (1) standardized the contributing watershed area by siting our study reaches at and above points on the channel where the watershed drained 2500 hectares, and (2) stratified the 10 100-m study reaches (on each stream) by Rosgen channel type. These first and second order study reaches were supplemented by monitoring of continuous channel stretches up to 2000 m in length at the other, larger streams.
Movement of debris pieces grossly mirrored streamflow magnitude. For instance, almost no pieces moved between surveys in 1993 and 1994. At two discharge gaging stations in the central Sierra, the maximum daily flow over the 1993-1994 winter was within the lowest 10% of daily maxima recorded at the gages. In contrast, the massive early 1997 daily maximum flows that were 30% greater than any other recorded flows swept away most of the tagged debris in several of the study streams. By 1997 most of the pieces originally tagged had moved in the study reaches of the Tahoe National Forest streams:
ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Mean ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ Percent
ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ bankfullÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ immobile
StreamÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ widthÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ pieces /1
Haypress CreekÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 12.3ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 1
East Fork CreekÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 8.2ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 2
N. Fork American RiverÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 9.1ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 5
Pauley CreekÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 12.0ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 9
Empire CreekÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 6.7ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 10
Badenaugh CreekÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 2.5ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 15
Sagehen CreekÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 3.3ÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ 39
1/ Pieces relocated in the same position they were initially tagged in 1991 or 1993.
Except for Sagehen Creek, the vast majority of the moderate and large woody debris in the study streams in 1997 either were transported in from upstream or entered the channel since the original survey from banks within the study reaches. These results suggest that the permanency of debris in Sierra streams can be low and that movement of debris can be frequent.
Two other conclusions are suggested by the mobility data:
1)Ý Mobility is generally greater on the streams with the larger bankfull widths (Haypress through Pauley), and
2)Ý The only two study streams on the east side of the Sierra, Badenaugh and Sagehen Creeks, had the lowest mobility values.
These two conclusions are potentially related given the generally drier conditions on the east side, with lower streamflows available for mobilizing debris.
In August 1994, the Cottonwood wildfire burned 46,800 forested acres on the Tahoe National Forest, including 99% of the Badenaugh Creek watershed. Burn intensity was high on 71% of the watershed and moderate on most of the rest of the watershed, and only a few of the trees in the lower portion of the study area had green growth in 1997.
Management of in-channel woody debris after wildfire is controversial. Post-fire increases in stream discharges can cause downstream flooding. The resulting heightened transport and accumulation of debris can wash out bridges and cause other damage. In-channel debris is often removed or cut into smaller pieces to expedite flushing through the system and to avoid debris jam formation. Biotic uses of debris for fish cover, triggering pool formation, storing sediment, and as food sources for invertebrates and micro-organisms are lost or reduced when debris is removed or cut up.
Because post-fire accumulation of woody debris is often blamed for downstream damages, we were particularly interested in the dynamics of debris accumulation through time after the fire in the Badenaugh watershed. We operationally defined debris aggregates as comprising at least four pieces all located within 1.5 m longitudinal distance, with at least one piece being equivalent in length to bankfull width. This length criteria is based on observations by Reid and Smith (The effects of hurricane Iniki on flood hazard on Kauai. USDA Forest Service.
Unpublished Final Report, December 7, 1992. PSW Research Station, Albany, California. p. 10) that '... jams are usually triggered by pieces that span the [channel] constriction' and by Piegay and Gurnell (Large woody debris and river geomorphological pattern: examples from S.E. France and S. England. 1997. Geomorphology 19:99-116. p. 112) that '...the size of pieces of wood may exceed the channel width ... so encouraging debris jams to develop.' We anticipated that aggregations would be larger (in both the number of debris pieces per aggregate and in debris volume per aggregate) and more prevalent after the fire. As a corollary to this presumption, we also anticipated that the total volume of woody debris would be greater after the fire as a source of elevated amounts of wood for the aggregates.
Fifty-seven percent of wood volume, and 25 percent of the pieces, were consumed by the fire. Many of the debris pieces that weren't completely incinerated were reduced appreciably in size by the fire with few post-fire pieces measuring over 6 m in length. The pre-fire mean length of 5.1 m decreased to 2.5 m after the fire. The percentage of pieces longer than the mean bankfull width decreased from over 65 before the fire to 33 after the fire. This reduction in length implies that fewer aggregates would form after the fire because there were fewer channel-spanning pieces available to act as anchors for the aggregates. The change in the size distribution of debris also potentially affects the stability of wood for habitat and geomorphic functions; fewer bank-spanning pieces imply less bank armoring by debris and a reduced role by wood as a cover element for fish.
Rather than increasing after the wildfire, the frequency of debris aggregations on the Badenaugh Creek study reaches decreased significantly from 2.0 per 100 m (channel length) in July 1994 to 1.2 per 100 m in September 1994 immediately after the fire and rose to only 1.4/100 m by 1997. The number of pieces per aggregate in the study reaches at Badenaugh Creek gradually increased over time (5.9 pre-fire, 5.5 immediately post-fire, 5.7 in 1995, 6.6 in 1996, and 6.9 in 1997). Mean aggregate volume did not increase after the fire on the study reaches in Badenaugh Creek and instead decreased significantly from 0.8 m3/aggregate pre-fire to 0.25 m3/aggregate immediately after the fire, with slight increases yearly to 0.51 m3/aggregate by 1997.
Contrary to convention wisdom, our findings do not suggest that debris jams increased after wildfire at the burned study stream. This is probably due to the shortening of jam-causing pieces by incineration. Decisions on the disposition of post-fire debris should consider the interaction between fire intensity, channel width and the size of the remaining wood. If few pieces of channel-spanning length remain post-fire they may pose relatively little downstream danger and potential debris removal should be tempered against the ecological benefits of retaining wood in the channels.
Results of this study through 1995 are described in 'Function and dynamics of woody debris in stream reaches in the central Sierra Nevada, California' (1998. Berg, Carlson and Azuma, Canadian Journal of Fisheries and Aquatic Science, 55:1807-1820).-
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