Big sudden runoffs over frozen ground will sweep a lot of manure and other organics into the river unintentially and this same thing can happen but it's usually less dramatic. My river just jumped 12 vertical feet last Saturday, and we might be seeing low O2 but all the fish died last summer anyway.
This very well could be the case. It is smack in the middle of central WI farm country. The resivior is a dam where the Big Eau Pleine River dumps into the WI River. The whole area is really neat. The fishing is usually good, the fishing on the WI is beyond good at the right time of year.
