LONG-TERM CHANGES IN HYDROLOGIC REGIME AND RIPARIAN ECOLOGY FOLLOWING IMPOUNDMENT IN A HUMID-CLIMATE WATERSHED

We analyzed the type of hydrologic adjustments resulting from flow regulation across a range of dam types, distributed throughout the Connecticut River watershed, using two approaches: 1) the Index of Hydrologic Alteration and 2) log-Pearson Type III flood frequency analysis. On average, maximum peak flows decrease by 32% in impounded rivers, but the effect decreases with increasing flow duration. 1-day minimum low flows increase following regulation, except for the hydro-electric facility on the mainstem. Hydrograph reversals occur more commonly now on the mainstem, but the tributary flood control structures experience diminished reversals. Major shifts in flood frequency occur, with the largest effect occurring downstream of tributary flood control impoundments, and less so downstream of the mainstem's hydro-electric facility. To test the implications of hydrologic alteration by dams for floodplain natural communities, we conducted a combined field and modeling study along two reaches and used a hydrologic model, HEC-RAS, to route pre- and post-impoundment discharges of different design recurrence intervals through each reach to establish corresponding reductions in elevation and effective wetted perimeter following post-dam discharge reductions. These riparian communities, which were flooded, on average, every 20 – 100 years pre-impoundment, were predicted to flood at 100 - > 100 year intervals, essentially isolating them completely from riverine influence. For floodplain forests, this alteration by impoundment suggests that even if other stresses facing these communities (human development, invasive exotics) were alleviated, this may not be sufficient to restore intact natural communities.