Wouldn't be cool if we could redirect river currents to wherever we want them, and then maybe place fish traps there to keep fish at a steady supply? Channel digging wouldn't be so hard a landscaping function now would it? I know this fits the trench digging idea, so I'll couple it with something relevant.
Fish Ponds, artificial Fish Ponds. Instead of just butchering fish we could place them in buckets of water for a while and then transport them to a fish pond to breed and create a steady and convenient supply of fish from. Fishponds should be constructible objects which we interact with, like chicken coops.