Case Study:
How Changes in Flow Affect a River System
The Hunt and Annaquatucket Rivers are small coastal streams draining less than 30 square miles each and flowing into Narragansett Bay along the westerly side of Narragansett Bay. These rivers are valuable as surface water resources for fishing, boating and aesthetic enjoyment and the underlying groundwater resources are extensively used for drinking water supply.
Groundwater in the Hunt-Annaquatucket-Pettaquamscutt (HAP) system is found in glacial stratified deposits and is in close hydraulic connection with the surface water system. Groundwater and surface water drainage boundaries are not coincident. Three public water suppliers withdraw water from the HAP - the Town of North Kingstown (10 wells), RI Economic Development Corporation (for Quonset) (3 wells) and Kent County Water Authority (1 well). Groundwater is also withdrawn at a privately owned industrial well and at three State-owned wells that provide water to the Lafayette State Fish Hatchery.
Nearly all the groundwater withdrawn from the HAP is derived from flow depletions in the surface waters that overlie the aquifer and water supply withdrawals have reduced flows in the Hunt River. Extreme low flows were seen at the Hunt River gage (USGS 01117000, Hunt River near East Greenwich, RI) between August 1 - 29, 2005 when flow in the Hunt River ranged from 1.1 - 4.5 cfs and again in the fall of 2007 where flows ranged between 1.1 - 4.5 cfs for more than two months (from August 19 - November 3 with the exception of 7 days in September and 3 days in October when the river responded to a day of rainfall).
The DEM Division of Fish and Wildlife (DEM, 2005) (DEM, 2007) conducted fish studies in the Hunt River in 1997, 2004 and 2007 to determine if increased water withdrawals have impacted fish communities. They examined fish at 9 stations, categorizing species into fluvial specialists (fish which require flowing water for the majority of their life cycle), fluvial dependents (fish which require flowing water for part of their lifecycle) and macrohabitat generalists (fish that do not require flowing water - pond fish). The 2004 studies document flow dependent species in the upper watershed including significant populations of native brook trout,suggesting high quality habitat. However, fish populations in the lower watershed in the Hunt River at Davisville and Potowomut show a shift from flow dependent species towards macrohabitat generalist species.
Freeman and Marcinek (2006) studied fish communities in piedmont streams in Georgia subject to various flow withdrawals. They found significant decreases in the number of fluvial species downstream of sites where half or more of the 7Q10 was withdrawn (The 7Q10 is a statistically derived flow value equal to the 7 day low flow that re-occurs once every 10 years). DEM compared the percentage of river fish and pond fish to the withdrawals from the watershed expressed as a percentage of 7Q10 and found a strong relationship. As the amount of withdrawals increases, the percentage of river fish decreases.
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Figure courtesy of RIDEM (V. Masson, A. Richardson)