Stream transient storage occurs where flow paths have a significantly lower velocity relative to the main channel. Transient storage zones have different biogeochemical characteristics compared to the main channel and have an important role in stream nutrient cycling and retention as a result. Previous studies have shown that biogeomorphic variation along stream channels can alter transient storage exchange, but the relative importance of geomorphic and vegetation-related factors varies. We are examining how geomorphic factors such as substrate type and channel morphology affect transient storage across seasons in Sycamore Creek, Maricopa County, Arizona. We performed hydrologic tracer (sodium chloride) injections in three reaches of this desert stream: (1) a riffle-dominated reach (“riffle”), (2) a pool-dominated reach (“pool”), and (3) a run-dominated reach (“run”). We monitored the tracer movement with conductivity sensors at two locations downstream of the injection point. Based on the patterns of conductivity over time, we used the one-dimensional transport model with inflow and storage (OTIS) to derive transient storage zone size and exchange rate. In order to contextualize transient storage variation among reaches, we conducted reach surveys of vegetation cover, flow type, substrate type, sinuosity, and main channel cross-sectional area.
The riffle reach had the highest transient storage exchange rate (0.065 s–1), followed by the run and pool reaches (0.040 and 0.024 s–1, respectively). The riffle reach also had the highest cross-sectional area of the transient storage zone (0.867 m2), while the run and pool reaches were comparable (0.490 and 0.504 m2, respectively). The presence of islands in the run and pool reaches was associated with a higher transient-storage exchange coefficient. The high cross-sectional area and exchange coefficient in the riffle reach were also associated with large substrate size (72% cobble) and a meandering stream channel (sinuosity=1.3). The run and pool reaches had less cobble substrate (40 and 0%, respectively) and straighter channels (sinuosity=1.0 for both). Our research suggests that streams with higher sinuosity and larger substrate particle size have larger transient storage zones and exchange, and may have greater capacity for nutrient cycling and retention as a result. Future injections will include more detailed vegetation surveys and will target stream reaches with variable sinuosity and substrate types.