For the purposes of keeping water, it wouldn’t be that useful.

The Klamath is far from major population centers, and is not currently part of any long-distance water transfer. (It’s been proposed but has actually been opposed by not only the local fisheries industry but by California.)

Also I think the grandparent is talking more about power generation; there needs to be a baseline level of water to generate power, and they already don’t generate a whole lot. Holding back water also means it isn’t getting used for power generation.

Dams can release water downstream as needed. Without them, water would just drain out to sea/ocean in the wet season, and in the dry season would be completely gone. Without dams, American southwest literally wouldn’t exist.

Yeah the part you're missing is that the area that controls the dam often wants to keep as much water as possible during times of drought, so there's often a local power struggle... this very dam has faced similar control debates.

Without a dam, during times of drought, the downstream party does not have water either. It is the very presence of the dam that enables considerations of this sort. Without it, the water just drains to the ocean quickly, and neither upstream nor downstream gets any water during dry season.