Salmon generally spawn in gravel beds in the extreme reaches of the Columbia and other large river systems on their return from the ocean - often hundreds of miles upstream from the river's mouth. After spending three to four years at sea chinook salmon return to spawn in the streams in which they were born and then to die. The South Fork of the Salmon River and the streams that feed it in the mountains of central Idaho are one such location.
River environment is critical in terms of both the development of salmon eggs and fry as well as the salmon's migration to the sea. This environment has changed for salmon returning to the South Fork over the last century and a half - changed in ways that have contributed to large decreases in the chinook population. Dams have obviously been the largest factor. They create barriers to fish both upstream and down. However, other factors have also played an important role. These include increased commercial fishing in the Pacific and in the river itself, changes in water temperature as a result of water pooling behind dams, and changes in water clarity brought about by both naturally occurring and man-made siltation. Fire, logging, mining, farming, and road construction and use have all contributed to increased silt in the rivers and streams of the Columbia system.
Eggs deposited and fertilized in nests in the river's gravel known as redds are dependent on the movement of water through the river's rocky bed to provide oxygen for development. Excessive amounts of fine silt in the stream can work its way into the redds and literally choke off the oxygen supply to the developing eggs.1
In 1987 - 1988 Idaho Department of Environmental Quality researchers studied the relationship between the amount of fine silt in the redds of chinook salmon and egg survival in the South Fork of the Salmon River. Examine the graph showing the results of their experiment.
Siltation at Poverty Flats on the South Fork of the Salmon River is pictured in a series of slides taken from a single photo point over the last 35 years. A graph of the levels of fine silt measured at the same location over the same time period accompanies the slides. Examine both the slides and graph and assess the suitability of the Poverty Flats spawning area.
Salmon fry begin their journey to the ocean after their first year in the streams where they were born. Water temperature is critical in both initiating their migration as well as to their ability to survive the journey. Water temperatures also impact the homeward migration of salmon returning from the sea.
The graph shows a baseline model of the water temperatures in 1972-1973 at the Lower Granite Dam on the Snake River upstream from its confluence with the Columbia. You can superimpose averages for the next four decades to study what has happened to average water temperatures over this time period.
1"South Fork Salmon River Spring/Summer Chinook Population," 2011, as found online at Salmon and Steelhead Recovery Plans for Idaho