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Coho salmon (Oncorhynchus kisutch) tend to spawn in small streams where the flow is 2.9–3.4 cfs.

Humans can impact the flow regime in three ways: controlling natural flow, increasing flow (adding water to the system), and decreasing flow (removing water from the system).

While potentially deleterious to the earliest life stages, flood events are essential to the development and maintenance of healthy stream systems. Floods change the stream structure by altering the active channel, creating new side channels, and recruiting and transporting large woody debris. Flood events bring new sediment to replenish stream banks, and fresh seeds and propagules to colonize open soils. High flows carry coarse sediment and deposit gravel in downstream reaches while flushing fine sediment from spawning gravel. While a severe flood year may result in a low survival rate for the present year cohort of salmonids, the resulting habitat changes can create improved habitat for future generations.

Streamflow is a key factor affecting the quality of salmon's freshwater habitat. Although the benefits to salmon for a specific increase in streamflows are difficult to assess precisely, biologists point to substantial scientific evidence that reductions in flows have contributed to the decline in salmon stocks throughout the region. Thus, a critical issue in the current policy setting will be how to maintain adequate streamflows to protect existing freshwater habitats and restore those that have been degraded.