Spec. Publ. Int. Ass. Sediment, 32(2002), 213-231

S.-J. MOSHER and I.P. MARTINI

Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

This paper illustrates the morphological changes produced in subarctic streams by regulated floods released from hydroelectric dams, and analyses sedimentologically the gravelly flood bars formed at the confluence of two streams in northern Ontario. Subarctic rivers have nival regimes, where strong floods occur during spring snow-melting and low flows for the rest of the year. Northern Ontario is a relatively flat region, and artificial reservoirs are not sufficiently large to retain the spring floodwaters nor, in most cases, can the waters be released through the turbines into the main stream. Instead, spillways are built, through which floodwaters bypass the hydroelectric stations, rejoining the main river downstream. In 1963, excess water in the headpond of the Mattagami River hydroelectric complex was spilled for the first time along the small (25 m wide and a few metres deep), meandering Adam Creek. Since then, regulated spring-flood discharges through the creek have averaged approximately 2100 m³ S-I, with a few floods exceeding 4000 m³ S-I. Adam Creek has experienced severe erosion along its lower reaches, which are underlain by Quaternary glacial deposits and poorly cemented, Mesozoic, clastic rocks. Approximately 52 x 106 m3 of sediment have been eroded and a canyon about 200 m wide and up to 30 m deep has developed. Approximately 2.5 x 106 m³ of this eroded material, predominantly the coarse fraction (boulders to coarse sand), have been retained in a junction bar and in three alternating side bars that have developed along a 5-km reach of the mainstream (Mattagami River) at and immediately downstream from the confluence with the Adam Creek spillway. The erosion of the spillway, the formation of the coarse-grained bars and the related local narrowing and deepening of the main river may have developed rapidly during the first few floods; subsequent floods have modified the surface structures (chutes, secondary channels, terraces) of the bars.

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