Haven't found anything near rivers personally but there's a stretch of muddy bank along the River Deben near me that I keep meaning to go check out properly after dark with a torch and camera. Been putting it off for months.
What kind of tracks are we talking though? Like size, shape, gait pattern? Because that stuff matters a lot if you're trying to rule out the obvious candidates first - otter, mink, deer coming down to drink, that sort of thing. I've seen some genuinely weird deer prints that looked almost bipedal at first glance because of how they'd sunk into wet mud.
Is anyone actually documenting these properly, plaster casts or decent scale photos? Would be good to see some actual evidence rather than just descriptions. Not trying to be dismissive, I just think cryptid track reports get written off so fast by mainstream people that we need to be really rigorous about this stuff if we want anyone to take it seriously.