Hyporheic exchange due to channel‐spanning logs

Abstract

The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive a large zone of river‐groundwater mixing, or hyporheic exchange. Flume experiments and numerical simulations show that river water downwells into the riverbed upstream of a channel‐spanning log and upwells downstream. Exchange rates are greatest near the log and decay exponentially with distance upstream and downstream. We developed equations for bed pressure profiles and hyporheic exchange rates in the vicinity of a channel‐spanning log that can be used to evaluate the impact of LWD removal or reintroduction on hyporheic mixing. The magnitude of pressure disturbance along the bed (and thus hyporheic exchange) increases with the fraction of channel depth blocked by the log and channel Froude number. Exchange rates are relatively insensitive to relative depth of the log (gap ratio). At natural densities, LWD in lowland streams drives reach‐averaged hyporheic exchange rates similar to a ripple‐covered bed. However, the length scales and residence times of hyporheic exchange due to LWD are greater. By removing LWD from streams, humans have altered patterns and rates of hyporheic exchange, which influence habitat distribution and quality for invertebrates and fish.