Overfalls

Overfalls are the steep, breaking, confused sea that forms where a strong tidal stream runs over an uneven bottom, a shoal, a ledge, or a sharp change of depth, or where two streams meet. The fast water lifting over the obstruction throws up short breaking waves and turbulence dangerous to small craft. Charts mark overfalls and tide rips with a distinctive symbol, and the sailing directions name the worst of them and the states of tide at which they form. They are worst at the strength of the stream around springs and ease off near slack water and at neaps. The navigator times a passage through a known race for slack, using the tidal stream atlas to predict it.