Hyporheic and Total Storage Exchange in Small Sand-Bed Streams

Abstract

Nutrient processing and carbon storage in stream ecosystems are linked to hydraulic retention. Hydraulic retention refers to the departure of stream flow from ideal “plug flow,” and reflects fluid movement through surface and hyporheic storage zones. Most existing information about hyporheic exchange is based on flume studies or field measurements in relatively steep streams with beds coarser than sand. Stream tracer studies may be used to quantify overall hydraulic retention, but disaggregation of surface and hyporheic retention components remains difficult. A stream tracer approach was used to compute the rates at which stream water is exchanged for water in storage zones (total storage) in short reaches of two small, sand-bed streams. Tracer curves were fit to the one-dimensional transport with inflow storage (OTIS-P) model. Networks of minipiezometers were used to measure hyporheic exchange. The rate of exchange between main channel flow and surface storage zones was assumed equal to the difference between total storage exchange computed by the modeled tracer curves and hyporheic exchange. This technique was used to measure the effects of flow obstructions (large wood) and differences in bed morphology and grain size on surface and hyporheic exchange. Parameters describing total retention were in the upper 50% of data compilations published by others that represent a range of stream sizes and morphologies, but hyporheic exchange was only 0.01% to 0.49% of total exchange. Retention did not correlate to differences in median bed material size, but increased with flow obstruction. Hyporheic storage was an insignificant component of total hydraulic retention in the small sand-bed streams studied.