This is a problem that the AD industry itself recognises.
Putting aside questions over taking land out of food production, the growing of maize for anaerobic digestion is never beneficial for rivers and usually quite damaging for them.
Without under-sowing, soil runoff can be very high, causing sedimentation of gravels and adding all the phosphorus, pesticides and other chemicals that comes with soil particles to rivers.
While growing maize for AD feedstock in sensitive areas of river catchments is not good practice, there are no rules against it. This is something that must be tackled by further legislation limiting feed crop growing or be tackled by farm advisors.