The Embankment Dam Phreatic Line – Where the water goes.
Earthen Embankment Dams are made to hold back water.. but only to a point.
It's a well known fact that ALL earthen dams leak to some extent. There is a well defined line of saturated earth that runs from inside the dam at the edge of the water to the outside of the dam, at the Toe of Slope where the outer slope of the earthen embankment meets the level ground below the water being impounded. This video explains why that line, called the Phreatic Line, is so important.
One of the key factors affecting the Phreatic Line, besides the intrusion of tree roots into it, is the Embankment Material & the Embankment Slope Ratio, or angle of the impounding berm itself. That is discussed in this webpage from IDC Technologies
This document states that dams slopes should be between 2:1 and 3:1 ration of dam base length to height. The Fairport Oxbow Lake Dam is 1:1 and worse in places. This should send alarm bells ringing in even the poorest mathematician.