Exploring the Influence of Morphologic Heterogeneity and Discharge on Downstream Transport in Streams With Multiple Logjams: 2. Insight From Numerical Models

Abstract

Logjams in streams create complex morphologic features such as pools, bars, and branching channels that enhance stream water storage and force water through the streambed. We examine how these features alter solute retention in stream corridors as discharge increases, backwaters expand, new channels activate, and jams become more submerged. Using numerical experiments informed by a well‐studied field site (Little Beaver Creek, Colorado, USA), we simulate coupled surface water and groundwater flow in stream reaches with logjams. Model results indicate that solute retention declines with increasing stream discharge, although the magnitude of decline depends on the metric used to quantify retention. We do not observe threshold‐like changes when dry channels reactivate but only modest deviations from the overall trend, suggesting that reactivation does not produce strong threshold‐like changes in retention behavior in already complex reaches. As discharge increases, backwater pools enlarge and hyporheic exchange volumes increase, but backwaters comprise a diminishing fraction of total channel storage, leading to reduced overall retention. Hyporheic zone volume and the turnover length both increase with discharge, indicating a larger but weakly connected hyporheic zone. Taken together, we expect substantially greater biogeochemical transformation at low flows in complex mountain streams. This research offers new insights into potential reach‐scale changes in solute transport through complex mountain streams as streamflow changes, which can be expected with the loss of snowpack under a warming climate.