Reference for Nedarim 54:3

דלמא אונסא דמיגליא שאני ומעברא מיגלי אונסיה

But why: surely he was unavoidably prevented? — Perhaps an accident that can be foreseen is different, and [the lack of] a ferry could be foreseen.<span class="x" onmousemove="('comment',' But the Mishnah refers to a river abnormally swollen by the rains and inciting snow. ');">³</span>

Jastrow