Thanks for the reply.

So, as I see it, so far I've worked out the

*decrease* in the entropy of the rivet.

The heat flowing into the reservoir from the rivet cooling will be the same as the heat loss from the rivet:

That gives me 17250 J.

Then, if we assume the temperature of the reservoir doesn't change, then, using the temperature of the river in Kelvins for T:

That gives me an

*increase* of entropy in the river as 62.1 JK

^{-1}
Finally, the change in gravitational potential energy is:

If we assume that this is all converted to heat in the river we can work out a second component for the

*increase* in entropy. I get S = 0.13 JK

^{-1}.

Overall this gives me a net increase in the entropy of the universe as S = 21.93 JK

^{-1}.

Is this the right track or have I completely messed up?