Follow up on this one: version 1.1.1 is up now. The four season chips beside the summit gauge show what a packet boat pays in each season, all year round: £14 in spring, £77 in summer, £24 in autumn and £14 in winter. So in spring you can already see what the water you're holding back is worth. The in game help names the chips, and the phone layout shows the same row.
We also ran the two measurements in the post above against our own code, and both hold. A reserve set under the dead line does nothing, and a line left at zero shuts the canal between day 23 and day 28. Both are on the fix list for the next update.