Transient storage and hyporheic flow along the Willamette River, Oregon: Field measurements and model estimates

Transient storage is a measure of the exchange of main channel flow with su bsurface hyporheic flow and surface water dead zones. Hyporheic flow, in wh ich river water enters the channel bed and banks to reemerge downstream, pr omotes biochemical processes that are important for water quality and aquat ic habitat. Previous studies have quantified transient storage and hyporhei c flow on small streams but were not specifically developed to identify bot h of these processes over long reaches of large rivers. We studied transien t storage on the eighth-order upper Willamette River, which flows through h igh-porosity gravel deposits conducive to hyporheic flow. We used main chan nel dye tracer studies and solute transport modeling to estimate transient storage on nine study reaches in a 26-km-long study area. We also took dye measurements within the transient storage zone to identify transient storag e Bow paths. We obtained estimates of transient storage exchange coefficien t, alpha (s) (mean equals 1.6 x 10(-4) s(-1)), and transient storage to mai n channel cross-sectional area, A(s)/A (mean equal to 0.28), that show that significant amounts of water follow flow paths with 0.2-30 hour transient storage zone residence times. Our dye measurements from the transient stora ge zone itself showed the occurrence of both subsurface and surface flow pa ths, confirming that hyporheic flow is an important component of estimated transient storage. We found that the two highest A(s)/A estimates were for reaches that spanned the only length of active main channel in our study ar ea that is unconstrained and where the river can rework large gravel deposi ts. Much of the natural channel complexity that historically promoted hypor heic flow no longer exists on the upper Willamette River. River management targeting the ecological functions provided by hyporheic flow might best fo cus on restoring historic hydrogeomorphic processes for creating sites cond ucive to hyporheic flow.