Any lic contractor would have been aware that you need a permit for ANYTHING they build.
Exactly, and any licensed contractor would know that to get that permit, you'd have to have erosion and sediment control measures spelled out in the application and/or depicted on the construction plans. Sounds to me like the eroasion control measures were either non-existent or woefully inadequate.
Which makes me wonder if he even used a licensed contractor to build his dam, or just hired some guy working off the tailgate of his truck who put in a low-ball bid.