Interesting question. I’m not familiar with the popular phrase or argument, “We are just letting the water go down into the ocean”. Indeed, some proportion of rainfall is returned quickly to the oceans as runoff enters streams and rivers that flow into the ocean. Shallow groundwater can also discharge into these streams or directly into the ocean. Some groundwater also gets trapped (at least temporarily) in “aquifers” -reservoirs of water in porous bedrock or sediments. Okay, so how does the water that goes into the ocean replenish our own water supply? The volume of fresh water in lakes, rivers, and aquifers in different areas can change drastically over short timescales (this is the problem at the center of the California drought), but the volume of water in the oceans remains relatively constant over short timescales (it does change due to changes in the mass of glacial ice caps). Water that drains off the continents into the ocean is eventually recycled back through the water cycle when ocean water evaporates, condenses, and falls as rain or snow. The water is distilled through the process of evaporation -almost all of the salt and other solutes are left behind. Approximately 90% of evaporated water that enters the atmosphere is from the sea surface (water.usgs.gov).

We can slow down the return of fresh water to the oceans by damming rivers and creating reservoirs, but there are environmental, ecologic, and geologic impacts to this. An important example of an ecologic impact is the disruption of the lifecycles of many kinds of fish that live most of their lives in the ocean but spawn in rivers. Some dams have features such as “fish ladders” that allow some fish to bypass the dams, but not all are equipped with such features. Dams also perturb the natural sediment transport of rivers. When flowing rivers enter dammed reservoirs, the velocity of the water decreases and sediments that were suspended in the water can fall out of suspension. This is why reservoirs need to be dredged to remove infilled sediment. Stretches of the rivers downstream of the dams become “sediment-starved”. The socio-political issues surrounding the damming of rivers are complicated. On one hand are the environmental impacts; on the other hand, damming of rivers provide sources of water that are important for agriculture and “clean” energy through hydroelectric power generation.