Wow. This is a surprisingly crappy answer to a common question in Sacramento.
Why conserve water when it all flows back to the river?
The answer is not gobbledy-gook about the hydrologic cycle. There are two parts to a better answer.
The letter writer describes indoor water use, which he perceives as a more-or-less closed system (river, pipes to him, use by him in sinks and showers and washing maching and toilet, back into pipes to the regional sanitation plant, treatment, back into river). No conspicuous loss at any point.
For residential indoor use, there are two good reasons to conserve. First, this year, we have so little water stored uphill of us that we need to make what we’ve got in Folsom Lake last through to the next rainy season. It could really run out in August. We conserve now to stretch supplies longer in time. Second, even if diverting through the homes and humans of Sacramento caused no loss, that water re-enters the system downstream, at the treatment plant. We don’t have a way to pump it back up to us. It might not be lost to the next downstream person, but it is lost to our use in the dry months to come. (Also, it takes energy to move it and use it and treat it twice, and fish are left with a tiny muddy trickle in the parts of the American and Sacramento Rivers between the Sacramento city pumps and the regional sanitation plant.)
But the real answer is that most of the water used by Sacramentans is used outside (rule of thumb is that 70%-80% of residential use is outdoor). That isn’t a closed system. Some gets lost to evaporation and transpiration when it gets spread an inch thick over lawns on hot days. Some may trickle into groundwater and it isn’t clear when that gets back to rivers. I heard someone say that only 56% of outdoor water in the Sacramento region returns to rivers. That was hearsay and I can’t even remember the source, so make of it what you will. A good blogger would try to find you a number, but I think we can all agree that I am only a pale shadow of my former glory. Still, my answer is a lot better than an answer that missed two fundamental points (that we’re trying to make limited supplies last longer in time, and that most water use is outdoors, which is not a closed system).