Spillway

This is a trout loch created by a man made earth dam. Such structures must have a spillway (normally off to one side) and the crest level here dictates the loch level.
While it doesn't look very formal this is the spillway; possibly in need of some remedial work with a pick and shovel.

A dam -even a small one- may be perceived as a simple structure but without careful site investigation, calculations, and reference to the experirnce of those who are specialists in this field there is an awful lot to go wrong. Over a certain storage capacity there are only a handful of engineers in our country qualified to sign one off.

Try doing a wee search on the Teton Dam in Idaho.