✔ 最佳答案
Because lack of water causes death faster than any other dietary deficiency.
In addition to drinking water, a river also supplies one with a means of transportation, an import/export route, an irrigation resource, and as your society develops, an energy resource (powering mills, etc.).
Let's not forget about waste management. It wasn't all that long ago when the basic alternatives to waste management were: bury it, burn it, or THROW IT IN THE RIVER.