Timing of upstream movement of adult salmonids varies by species and stock (Table 28, Figure 5 will be added). The great majority of pre-spawners migrate upstream in fall and winter. After the usual summer drought, rain storms arrive in the fall or winter, causing stream flows to increase. This greatly improves conditions for upstream migration and the fall runs begin migrating upstream to spawn. Migration also occurs during winter and early spring. In April and May, spring chinook and spring steelhead move upstream.

Timing and spatial distribution of upstream migration and spawning is limited by the timing and location of migration barriers. Barriers to migration occur through the interaction of topography, human structures, and the timing of precipitation and high stream flows. Potential barriers to adult upstream migration include falls, cascades, culverts, low flows, and high water temperatures. Some topographic features such as waterfalls are impassable at all levels of flow. Other sections of the stream become barriers during low flows or high flows. Features such as cascades and culverts may become impassable during high flows due to increased water velocity and during low flows due to lack of water depth. Each species has different swimming and leaping abilities so that a barrier for one species may be passable by others.

Although the lower Smith River remains accessible to returning adult salmonids during the low flows of summer, access to streams in the upper watershed is poor during this time. Barriers to migration during low flows are most likely in aggraded areas. In these areas, a portion of the stream flow travels subsurface through the gravel, and the depth of water in the channel may be inadequate for fish passage. For example, when aggradation occurs at the confluence of a tributary, fish access to the tributary may be blocked.

Different anadromous salmonid species have different swimming and jumping abilities (Table 31). Therefore, species vary in their ability to access tributaries in the river network, especially in high gradient streams. In addition, accessibility of the streams for each species varies annually due to differences in quantity of stream flow. Steelhead are strong swimmers and are able to use higher areas of the watershed. They also can spawn in smaller gravel than other salmonid species. There are many steep streams in the headwaters of the Smith River watershed which only steelhead can access due to their superior swimming and leaping abilities. Thus the Smith River has a high capacity for production of steelhead. Coho salmon have difficulty in accessing streams with gradients greater than 3% due to limited swimming and leaping prowess (Trush 1995, Bjornn and Reiser 1991, Table 31). Therefore, coho utilize a limited portion of the Smith River system consisting of low gradient tributaries. They are found in tributaries of the lower river and to a lesser extent low-gradient upper tributaries (Scriven in press, Figure 4 will be added). Chinook have greater swimming and leaping abilities than coho but less than steelhead (Table 31). Because they prefer large gravel up to six inches in diameter, chinook are able to spawn in the mainstem and larger tributaries. The small run of chum salmon in the Smith River (Waldvogel 1985, 1988) probably spawns exclusively in lower tributaries.

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