LONG-TERM ASSESSMENT OF LOWHEAD DAM REMOVAL OR MODIFICATION AND ITS EFFECT ON URBAN STREAM QUALITY, SPRINGFIELD, OH
Water quality, sediment, and macroinvertebrate sampling in an upstream reference riffle, the impounded or formerly impounded area, and in the recovery riffle immediately downstream of the impacted area have periodically been collected and used to calculate index values for pollution tolerance, biodiversity, and habitat. Comparison of these stream health indices have been complicated by differences in flow conditions (i.e., average flow stage, timing, number, and frequency of stormflow events), but streamflow in 2016, seven years after modification to the first dam, provided an opportunity to assess stream health under flow conditions similar to 2009, prior to modification. The data indicate that coarser bed material improves dissolved oxygen content and is associated with lower pollution tolerance (higher pollution tolerance index value) and greater biodiversity of stream macroinvertebrates. Lowhead dam removal and modifications do not improve stream health where they maintain or create impounded flow conditions, particularly in exceedingly wide stream reaches typical of that upstream of lowhead dams. Flow constrictions designed to create the recreational hydraulic can impound both baseflow and stormflow, similar to the lowhead dam, because of the storage of finer sediment. Creating a two-stage channel upstream of these structures, restoring channel width so that it is consistent with the current river regime, should mitigate this.