The role of large woody debris in forest streams has become much better understood over the last two decades. Research has shown that large woody debris improves fish habitat by increasing pool types and sizes, sediment storage, and local scour. However, simply knowing that "wood is good" is insufficient for making site-specific prescriptions that include the number and size of woody debris pieces required to maintain adequate aquatic habitat. The goal of this project is to help provide such information for Coast Range headwater streams by investigating the effects of the size and orientation of large woody debris pieces on debris stability, stream channel morphology, and aquatic habitat.
Woody debris of three different sizes--8, 16, and 24 inches in diameter--were placed in two cobble-bedded, sandstone coastal headwater streams at two different orientations: "spanners" were placed perpendicular to streamflow and resting on the bottom of the channel, and "ramps" were oriented downstream at approximately 45 degrees to streamflow. Both spanners and ramps were free to rise during high flows. Thirty-six debris pieces were installed during the summer of 1989.
Data collection consisted of inventories of both fish habitat and fish population. Fish habitat inventories identified stream reaches as pools, glides, or riffles and measured their width, length, and depth. A census of fish populations was taken by electro-shocking. A topographic map of the stream channel was made so that changes in channel morphology, such as local scour and fill, could be monitored.
Data have been collected and analyzed for the project through the summer of 1993. The latest results are summarized in COPE Report 7(4):3-6. The results show clearly that the addition of large woody debris had an effect on local scour and pool development. The results focus on residual pool volume--the volume of water that would remain in pools in a stream if surface flow should cease. Residual pool volumes are determined from the topographic maps of the streams. The two study streams, J-Line Creek and Preacher Creek, had increases of 679 percent and 304 percent, respectively, in residual pool volume between 1989 and 1993. In J-Line Creek, total residual pool volume went from 1.1 m3 in 1989 to 7.7 m3 in 1993. In Preacher Creek, residual pool volume went from 2.5 m3 in 1989 to 7.9 m3 in 1993.
Because residual pool volume is also caused by naturally occurring woody debris and non-debris structures such as boulders, not all of the increase in residual pool volume can be attributed to the treatments. In Preacher Creek, the residual pool volume not associated with the treatments is 4.4 m3, or 57 percent of the total, and in J-Line Creek it is 2.2 m3, or 29 percent of the total. Thus, 43 percent of residual pool volume in Preacher Creek and 71 percent in J-Line Creek is directly attributable to the large woody debris added as treatments to the streams. Figure 10 shows the distribution of residual pool volume attributable to all factors including treatment debris.
The most effective treatment for developing residual pool volume was adding large spanners. Of the residual pool volume attributed to treatment debris, 90 and 84 percent in J-Line Creek and Preacher Creek, respectively, were associated with the 24-inch spanners.
The data clearly show a treatment effect on densities of older cutthroat trout (i.e., excluding the young of that year). Pools with large spanners (16 and 24 inches) have cutthroat trout densities similar to those in naturally occurring pools, while the other treatments have cutthroat trout densities similar to those in riffle and glide habitats (Figure 11). Most trout were found in pools associated with the 24-inch spanners.
Densities of juvenile coho, steelhead, and young-of-year trout were highly variable, and no clear trend was obvious. Other factors such as interactions with other populations or mortality associated with migration and ocean conditions appear to have overwhelmed any treatment effect.
Results from this project should be helpful for predicting habitat improvements when debris is added to streams as a part of enhancement projects and for modeling the effect of natural recruitment of riparian trees. This project should also provide information for writing site-specific prescriptions for streams and riparian areas directed at improving aquatic habitat. For example, the preliminary results from this project clearly show that if the objective is to affect local scour and develop pools for summer habitat in cobble-bedded, sandstone headwater streams in the central Oregon Coast Range, an effective way to accomplish this is to install large logs perpendicular to the stream flow and flush with the stream bottom.
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