POOL MORPHOLOGY IN GRAVEL-BED STREAMS IN THE BLUE MOUNTAINS OF EASTERN OREGON: LONG-TERM CONTROLS VS. SHORT-TERM (HUMAN AND BIOTIC) INFLUENCES

Much management effort has been directed toward pool morphology, which is a key characteristic for both channel geomorphology and fish habitat. Targets or standards have been developed for pool dimensions, and stream restoration activities are typically aimed improving at pool morphology and other closely linked characteristics such as cover, temperature and spawning gravel quality. Management strategies and standards have often failed to distinguish between channel characteristics influenced mainly by controls that vary over short time scales, such as human influences and large wood loading, and those influenced by controls that vary only over long time scales. Standards should be stratified by long-term controls. To what extent is pool morphology determined by long-term controls (drainage area scaling, gradient, bed and bank material) vs. controls that vary over shorter time scales (particularly human and biotic influences on sinuosity and gradient, w:d, and large wood)?

Detailed reach measurements were made at over 20 forest and meadow sites east of the Cascades. The streams are all alluvial pool-riffle channels, with drainage areas between about 15 and 100 km2 and valley gradients of 0.004 to >0.03. The sites exhibit a large range of sinuosity (1.05 to 1.9) and channel width, reflecting combinations of long-term controls and past human influences such as straightening and overgrazing. At most sites wood loading is low, although wood has been added at some sites as a restoration strategy. Pool characteristics reflect a mixture of short- and long-term controls. For example, pool area generally increases with decreasing gradient and increasing sinuosity, but human-caused degradation produces low outliers while addition of large wood and beavers shift values higher. Pool length, spacing and depth show similar multivariate relationships.