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?