Using "Residual Depths" to Monitor Pool Depths Independently of Discharge

As vital components of habitat for stream fishes, pools are often monitored to follow the effects of enhancement projects and natural stream processes. Variations of water depth with discharge, however, can complicate monitoring changes in the depth and volume of pools. To subtract the effect of discharge on depth in pools, residual depths can be measured. Residual depth is the difference in depth or bed elevation between a pool and the downstream riffle crest. Residual pool depth or volume can be measured at wadable flows by using only a tape and graduated sounding rod. Residual dimensions represent extreme low-flow conditions, which often determine the capacity of streams to produce fish. The measurement of residual depth is an unbiased way to easily distinguish pools from other reaches. Its application is illustrated by a case study on a stream in northern California.