firecrackers
I agree. This is going to require some non-traditional thinking.
Use noise, or perhaps you can obtain a bottle of the scent of a natural predator, or something like that. Or a scarecrow. Someone mentioned placing a barrier between the grasses and the water: you can do that with temporary orange plastic snowfence and some stakes.
I remember as a kid being in a cherry orchard that used loudspeakers with random popping and cracking sounds to scare away blackbirds.