Time scale of recovery of hyporheic complexity and denitrification after stream channelization or maintenance events

In agricultural streams, channelization and maintenance events often remove accumulated coarse-grained sediments that provide hyporheic function.  To test how rapidly hyporheic denitrification recovers, we instrumented the hyporheic zone of one riffle in streams that underwent channel maintenance 4,  7, and 30 years ago.  We observed that decreases in hyporheic nitrate concentrations (and increased geomorphic complexity) are correlated with increasing time since maintenance.  Bromide injection tests show hyporheic residence time in the riffles ranging from 10 to 30 minutes in all three streams.  However, tracer dilution at the base of the riffles indicate another water source.  Since the major anion chemistry through the riffle doesn’t change, we conclude that the dilution comes from stream water flowing along a hyporheic path with a residence time longer than the tracer test.  In the streams where maintenance occurred 4 and 7 years ago, no or minor denitrification appears to be occurring along the long hyporheic flow paths.  After 30 years, denitrification along the long flow paths results in up to a 75% decrease in nitrate concentrations in the riffle hyporheic zone.  Our results suggest that lengthening the time between channel maintenance events may increase the amount of denitrification in modified streams.