Geomorphological and botanical change on the Outer Slave River Delta, NWT, before and after impoundment of the Peace River

Abstract

This paper examines geomorphological and botanical changes on the outer low elevation portion of the Slave River Delta prior to and after impoundment of the Peace River at Hudson's Hope, British Columbia. The Slave River Delta deposits are approximately 8300 km2, about 5% (400 km2) is defined as the active delta which progrades into Great Slave Lake through an active system of distributaries. This active portion of the delta is the area studied. The Peace River provides approximately 65% of the annual flow of the Slave River. Alteration of the natural flow regime and suspended sediment load of the Slave River has resulted from the upstream impoundment. Calculations indicate that the annual suspended sediment load in the Slave River has decreased by 33%. Using aerial photographs of the Slave Delta taken in 1946 and 1966, prior to damming the Peace River, geomorphological change and the spatial changes in plant assemblages have been recorded using GIS software. Aerial photographs taken in 1977 and 1994 were used in similar fashion to document geomorphological and botanical change and rates of change after impoundment. The Outer Delta is most susceptible to hydrological and suspended sediment changes in the river system. As such it is shown that geomorphological development in these Outer Delta landforms has been notably reduced since impoundment and changes in plant assemblage distribution indicate a drier, less productive environment. Results of aerial photo analysis show that the rate of development of distinctive cleavage bar islands on the Outer Delta has been notably reduced since impoundment. Changes in plant assemblage areas on these islands indicates both an autogenic and allogenic change to a drier, less productive environment and actual loss, by erosion, of some of the most productive Equisetum assemblages. © 1997 John Wiley & Sons, Ltd.