These deposits form an ever-shifting layer, lying loosely over a mantle of hard basalt. Much of the water in the river travels through this aquifer providing maximum oxygenation for the spawn of anadromous fish. The Sandy River basin contains vast areas of spawning gravel for salmon and steelhead. Wild steelhead return to the Sandy River every month of the year. Hatchery Steelhead are available approximately eleven months each year.
Chinook salmon are available in reasonable condition and catchable numbers six months each year.
The Sandy River though still a guarded secret among fly fishing enthusiasts, has the best year round run of steelhead in the region. Its geographic location, topography and geologic history make it the perfect steelhead factory. A careful blending of both hatchery and endemic stocks, bring bright steelhead year round. Probably at least five genetically different races of steelhead ascend the Sandy river each year. This steelhead fishery is combined with the added bonus of Chinook and Coho salmon and a budding resident trout fishery.