Semerwater is the largest natural lake in North Yorkshire, England. It is half a mile (800 m) long and in the heart of Wensleydale.

According to local legend, where Semerwater now lies among the hills a proud city once stood. To it one evening there came an old man, poor and hungry, and begging food from door to door - but in vain. Close by lived a lonely shepherd and his wife, and it was in their cottage that the wanderer at last found food and rest. In the morning he rose and, facing the town with arms uplifted, pronounced this curse: "Semerwater rise! Semerwater sink! And swallow the town, all save this house, Where they gave me meat and drink." Immediately, the waters rose, drowning the inhospitable town.