Responsa for Eruvin 176:19

התם מאי ניחוש לה אי משום קילקול הא מיקלקלא וקיימא אי משום גזירה שמא יאמרו צנורו של פלוני מקלח מים סתם צנורות מקלחין הן

surely, [it may be retorted,] it is any case spoilt;<span class="x" onmousemove="('comment',' By the rains.');">⁴⁷</span> and if against the possibility of the assumption that So-and-so's gutter was spouting water,<span class="x" onmousemove="('comment',' On the Sabbath: in consequence of which people might allow themselves to carry also directly from a private into a public domain.');">⁴⁸</span> all gutters, as a rule, spout water.<span class="x" onmousemove="('comment',' On a rainy day. People would assume the water to be rather the accumulated rain water than the lesser quantity of waste water. In the case of a drain in the dry season, however, people observing the flow front a private into a public domain and knowing full well that it was the result of human action, might well come to the conclusion that the carrying of objects from the one domain into the other is also permitted. Hence the preventive measure.');">⁴⁹</span>

Teshuvot Maharam