A small (.5 HP) submersible pump in the canal set on a timer for about 6 hours a day would probably solve the stagnation. You could point the discharge to be just in the beginning of the canal to create a clockwise rotation. At a 20' width you could get quite a swirl/lazy river effect going. And a small, bottom diffused, aerator would eliminate any leaf debris concerns right away too.

On a safety note, those sides are pretty sttep if someone falls in and has to get out fast. A 5-6' depth would be better at preventing weeds.