Commentary for Bava Metzia 215:10

נהרדעי אמרי אפילו משום דינא דבר מצרא מסלקינן ליה משום שנאמר (דברים ו, יח) ועשית הישר והטוב בעיני ה'

those dwelling on the lower reaches must aid the upper inhabitants, but not vice versa.<span class="x" onmousemove="('comment',' If there are obstacles on the upper parts of the river, the water flow is adversely affected for the lower too. But on the other hand, there is no profit for the upper inhabitants to clear the lower portions, for the greater the ease with which the water runs downwards, the less water is left for them. ');">⁹</span>

Rashi on Bava Metzia