The number one issue with earthen dams has always been keeping water inside them. Because of the materials used in earthen embankment design & construction, mainly natural earthen materials of various types, these type embankment dams become prone to over-saturation at some point... and then they start to leak.
Once the embankment materials become too wet the water can once again seek lower ground by seepage and piping through the earth making up it's impounding wall. It will use small cracks in materials, or roots of larger vegetation if available, and work it's way from the top level of the water pool to the lowest point of the dam embankment, called the Toe.
Normally there will be a Toe Drain of some sort designed into the embankment, maybe a ditch or stone drainage, that will catch seepage water from the dam and deliver it to a local storm drain system to be recollected and used. It could also be diverted to the many canal culverts that allow natural streams & rivers to pass under the waterway, through the embankment near the Toe. This drain, and the amount of water flowing through it, is the first indicator of a potential dam safety issue, it's the warning system indicating a potential leak or other issue with the earthen dam.Being able to properly INSPECT & MONITOR the amount of water in these Toe Drains, along VISUAL inspection of the Embankment Slope are keys to keeping earthen dams SAFE, like our raised earthen embankments lining the Erie Canal.
Here's a great set of videos explaining about Dams, Seepage & Piping from one of the leading experts in Dam Design & Construction, Dr. Ralph B. Peck
Dr. Peck's Bio can be found on Wikipedia
His work is used by FEMA in it's EXPERT VIDEO SERIES
He has a dedicated website to his work in GEO-ENGINEERING
Dr. Peck understood the benefits of Dam Modelling to predict failure
These videos are a bit long, almost 40 minutes each. Part 2 has more on why Seepage, Piping & Proper Inspections are critical to dam safety. Part 1 is about a whole host of historical Dam Failures and why they happened.
There is a dam in Part 1 that was made of earth, similar to the Erie Canal Embankments... Dr. Peck shows what happened as the Dam failed, but not fully. The leak was caught by a Canal Inspector who happened by on a holiday walk... he saved thousands by spotting running dirty water in the Toe Drain
Piping & Seepage Part 1