Tarquin
The Idle is not in the Fens so your comment re: untold damage is not relevant.
If marigolds require water to survive then they should be relocated to existing marshland. It is not our responsibility to create artificial marshes just to suit an erroneous flower.
Deliberate flooding of surrounding fields in springtime sees the water temperature in the fields rise disproportionately to the main river course. This encourages the fish to move into the warmer water and inevitably triggers spawning. Consequently we lose a whole year's progeny because they get left behind in pools as the levels drop.
This is not supposition, it's a fact as any of the anglers who have been involved in the subsequent fish rescues will tell you. Predators have a field day with the mature roach, bream, chub and dace. Large numbers of carp to over 24lb, bream to near 7lb, roach to 2.5lb, chub to 5lb, etc., have been rescued and returned but many are missed.
The term decimate is often used wrongly. It means to reduce by a tenth. It's a fair assumption to say that the river is genuinely decimated by these wolly hatted actions. When that happens ten years in a row the impact is huge.
Fish more important than marigolds? I say yes. You can grow bluddy great marigolds in a very small garden if you really want to. Try that with fish.
What next, let's recreate dynasaur habitat, just in case? Rubbish. If you want to mollycoddle rare plants, create the right environment for them in managed, self contained areas. Please don't tell me that it's okay to decimate entire fish populations for the sake of a few precious flowers.
Sorry mate, I don't agree!