Narrows

A narrows is a restricted land or water passage, most often taking the form of a strait, though it can also be a water gap. It commonly develops where a stream cuts through a tilted bed of resistant rock sandwiched between softer layers; where that hard rock stays standing as the surrounding softer rock erodes away, a narrows forms and acts something like a dam, raising the water level for some distance upstream. Narrows are ecologically and practically significant, serving as good places to catch migrating fish and as an important site for wind-driven mixing of water, a process whose cooling effect can form ice even when the surrounding air is above freezing.