Nelson, Kurt. 1998. The influence of sediment supply and large woody debris on pool characteristics and habitat diversity. M.S.
The influence that sediment supply and large woody debris (LWD) has on pool characteristics and habitat diversity was examined in streams within the Skagit and Stillaguamish Watersheds. The study focused on streams with channel slopes of 0.5% to 3.1% and basin areas between 8.6 km2 and 33.8 km2. Linear regression and analysis of variance were used to assess relationships between sediment supply and LWD and the following habitat characteristics: residual pool depth, pool spacing, percent pool, and habitat diversity. An index of habitat evenness, based on habitat unit area, was used to describe habitat diversity. Sediment supply estimates from mass wasting inventories were used to describe sediment supply conditions within the stream reaches studied.
The results of this study suggest that sediment supply influenced bankfull channel width and width-to-depth ratio. The effects were most pronounced in unconfined to moderately confined channels. The effect that sediment supply had on pool characteristics and habitat diversity was weak or subordinate to other basin and channel characteristics. The only two pool characteristics that appeared to be affected by sediment supply were residual pool depth and pool frequency. The significance of the effect sediment supply has within a reach was largely controlled by local geomorphic controls or obstructions.
Residual pool depths appear to be predominantly a function of basin area (r2= 0.50, P is less than .001). However, when sediment supply was included in the relationship, basin area and sediment supply together described 69% of the variation observed in residual pool depth. Large woody debris had a significant influence on both pool area (r2= 0.45, P is less than .002) and the frequency of pools (r2= 0.66, P is less than .001). As LWD increased in abundance, the frequency of pools and pool area increased. Large woody debris that was most effective at forming pools was wood that was in contact with the summer low flow channel. Large woody debris was associated with 58% of the measured pools. The index of habitat evenness was positively related to substrate characteristics (i.e., D84), LWD, and bankfull channel width.
Four relationships observed during this study may have management uses. Those relationships include; (1) basin area and residual pool depth, (2) bankfull channel width and sediment supply, (3) width-to-depth ratio and sediment supply, and (4) LWD to pool spacing. Development of these relationships by basin or region may provide a useful technique to evaluate or screen channel reaches, and predict channel conditions.