Transient storage assessments of dye-tracer injections in rivers of the Willamette Basin, Oregon

Rhodamine WT dye-tracer injections in rivers of the Willamette Basin yield concentration-time curves with characteristically long recession times sugg estive of active transient storage processes. The scale of drainage areas c ontributing to the stream reaches studied in the Willamette Basin ranges fr om 10 to 12,000 km(2). A transient storage assessment of the tracer studies has been completed using the U.S. Geological Survey's One-dimensional Tran sport with Inflow and Storage (OTIS) model, which incorporates storage exch ange and decay functions along with the traditional dispersion and advectio n transport equation. The analysis estimates solute transport of the dye. I t identifies first-order decay coefficients to be on the order of 10(-5)/se d for the nonconservative Rhodamine WT. On an individual subreach basis, th e first-order decay is slower (typically by an order of magnitude) than the transient storage process, indicating that nonconservative tracers may be used to evaluate transient storage in rivers. In the transient storage anal ysis, a dimensionless parameter (A(S)/A) expresses the spatial extent of st orage zone area relative to stream cross section. In certain reaches of Wil lamette Basin pool-and-riffle, gravel-bed rivers, this parameter was as lar ge as 0.5. A measure of the storage exchange flux was calculated for each s tream subreach in the simulation analysis. This storage exchange is shown s ubjectively to be higher at higher stream discharges. Hyporheic linkage bet ween streams and subsurface flows is the probable physical mechanism contri buting to a significant part of this inferred active transient storage. Hyp orheic linkages are further suggested by detailed measurements of river dis charge with an Acoustic Doppler Current Profiler system delineating zones i n two large rivers where water alternately enters and leaves the surface ch annels through gravel-and-cobble riverbeds. Measurements show patterns of h yporheic exchange that are highly variable in time and space.