Impacts of Land-Use Changes and Hydrologic Modification on the Lower Boise River, Idaho, USA
Dorene MacCoy and David Blew
Abstract.—In less than two centuries, the lower Boise River below Lucky Peak Dam in southwestern Idaho has been transformed from a meandering, braided, gravel-bed river that supported large runs of salmon to a channelized, regulated, urban river that also provides irrigation water to more than 1,300 km2 of land. The construction of three large dams in the upper basin dramatically altered the flow regime and sediment supply to the lower river. Flows are no longer sufficient to mobilize bed sediments and have allowed cottonwood trees and alien hardwoods to stabilize parafluvial surfaces, thereby narrowing sections of the river channel. Cadastral survey notes of 1867 and 1868 were used to recreate features associated with the lower Boise River Valley and identify characteristics of the river channel prior to dam construction and urbanization. Gravel and sand bars, historically present throughout the river, which are necessary to maintain biodiversity and productivity, are currently scarce. Sloughs were a dominant feature on the floodplain of the late 1800s, but today have been converted to irrigation canals, drains, or residential and commercial land uses. Flow alterations, water quality degradation, and habitat loss due to urbanization near the lower Boise River have resulted in macroinvertebrate and fish assemblages dominated by tolerant and alien species.