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The Klamath River is a good example because when George Bush cut off the flow of the Klamath, large numbers of Salmon died in the lower river.

A similar situation arose in Texas in 2009 when the flow of the Guadalupe River was cut and a couple dozen whooping cranes died.

The issue is about minimum flows.

Some say that any drop of water reaching the ocean is wasted water. Others say that there has to be a minimum flow, but they may not be able to agree as to exactly how much the minimum flow should be.

Often this is covered by water rights such a Reserved Water Rights.

OTOH, under prior appropriation water rights there is the "highest beneficial use" clause and traditionally agriculture was/is the highest beneficial use and economic output of the oceans is not considered. And in modern times, water reserved for recreational use has a higher economic output than some ag products. Utilizing water for golf courses or trout fishing generates more economic output than using the water to grow a low value crop like hay or potatoes.

In CA the minimum flow on the rivers varies a lot with the northcoast rivers having higher minimum flows than the southern rivers. Often that will be because they are designated as Wild and Scenic.

Minimum flow is a volume amount but the quality of the water affects it and rivers carrying a lot of ag chemicals result in dead zones in the coastal waters.