Downstream Effects of Water Withdrawal in a Small, High-Gradient Basin: Erosion and Deposition on the Skokomish River Delta

This paper analyzes downstream effects of freshwater flow diversion from a small, active-continentalmargin river basin. The Skokomish River delta, a tributary estuary to Hood Canal in Washington state, receives drainage from the southeastern side of the Olympic Mountains. Its drainage basin is steep, and rainfall is high. Since completion of two dams in 1930, approximately 40% of the annual average runoff of the entire system has been diverted from the North Fork Skokomish River for power production; this water does not pass through the lower river or over the delta. Extensive logging has occurred in the remainder of the basin. Comparison of prediversion (1885) and postdiversion (1941 and 1972) bathymetric surveys suggest that deposition (about 0.013 m yr-1 to 0.022 m yr-1) has occurred on most of the inner delta and erosion (up to 0.011 m yr-1 to 0.033 m yr1) on much of the outer delta. More rapid postconstruction deposition occurred within the river mouth itself, where the 1926 to 1941 deposition rate was 0.04-0.11 m yr-1. Nine of 12 historical bathymetric change cross-sections show steepening of the delta surface, two are neutral, and one shows aggradation. This steepening has apparently been caused by a loss of sediment transport capacity in the lower river and estuary combined with steady or increased (due to logging) sediment supply. Although the total area of unvegetated tidal flats has decreased by only about 2%, there has been a 15-19% loss of highly productive low intertidal surface area and an estimated 17% loss of eelgrass (Zostera marina) habitat. A reduction in the size of mesohaline mixing zone has also occurred. These habitat losses are similar to those observed elsewhere in the world in larger river basins that have suffered water withdrawals of the same magnitude, but their impacts either cannot be evaluated or understood casually through consideration of simple measures like changes in total estuarine deltaic area. Evaluation of estuarine effects of anthropogenic modification must, therefore, include consideration of both changes in habitat function and in the physical processes. These must be evaluated within the totality of the river basin-estuary system that cause these changes. In this case, sediment transport constitutes the critical link between fluvial alterations and the remote downstream, estuarine consequences thereof. Introduction and Regional Setting Freshwater flow diversion and regulation for purposes of flood control have caused serious damage to estuaries worldwide (Clark and Benson 1981; Rozengurt and Haydock 1981; Rozengurt and Hedgepeth 1989; Simenstad et al. 1992). Most studies of this problem have focused on large drainage basins in arid regions where the most extreme diversions have occurred. The intent of this paper is to examine changes caused by diversion 1 Corresponding author. 2 Present address: Oregon Graduate Institute, Center for Coastal and Land-Margin Research, Beaverton, Oregon 97006; tele 503/690-1372; fax 503/690-1273; e-mail djay@ ccalmr.ogi.edu. of 40% of annual average flow from a small system situated in a region with a wet climate, the Skokomish River and estuary in Washington state (Fig. 1). It is argued herein that a loss of sediment transport capacity has caused erosion on the outer Skokomish delta combined with shoaling on the inner delta and in the lower part of the mainstem river tributary to the delta. Steepening of the delta has caused a substantial loss of low intertidal habitat and eelgrass (Zostera marina). A loss of primary productivity and compression of the mesohaline mixing zone have also likely occurred. These impacts are comparable with and similar to those in larger basins with similar percentages of water withdrawal. Alterations to the sediment transport regime arising from flow regulation and diversion ? 1996 Estuarine Research Federation 501 502 D. A. Jay and C. A. Simenstad