I wouldn't worry about the condition of the river. The Missouri is brown because of sediments though I'm sure there's a fair amount of suspended nutrients from upstream wheat farms. The river is still cleaner than many in the Midwest as riverfronts in Green Bay, Milwaukee, and Cleveland have rebounded to being destinations over the past 30 years.
Was more making the point that towns in the area really don't have riverfronts, and speculating it might be because the river is kinda gross. I mean, admittedly I don't know Omaha well but I don't think of the river as a trendy destination district there.
Modifying this to say that what the towns do have along the Missouri is parks. Going north from KC almost every decently sized town has a riverfront park, with the amount of use depending on the town (not directly correlated with size, either). I don't think there's a "riverfront" district between KC and Great Falls, MT. This doesn't necessarily prove that a redevelopment district won't work, but it would be going against the grain for the area.
Also for what it's worth my Nebraskan wife thinks it won't work either... we both could be wrong.