Question
We have a huge back yard (4 acres)situated on the Red River that divides Fargo ND and Moorhead MN. Most years, the lower 1/4 of the lawn is flooded as the Red goes over its banks. Are there any perennials that I can plant that will tolerate the soggy conditions in the spring, followed by a hot (85-100 degrees)dry summer? The area that is flooded usually needs to be reseeded, and I would like to have flowers or plants that visually block the area as the grass is starting to grow.

Answer
Extreme conditions -- tricky, but not impossible.  This will be one of those projects where the outcome will be decided by trial and error.  Try a few specimens and see what survives.  Select those you like best and stock up on them.

I am partial to Iris.  Especially Siberlian Iris.  So I'll begin with those, because they require a great deal of constant moisture and I am inable to provide that on a regular basis.

Specialist Burns Water Gardens posts photos and care at its website on Iris that fit your description:

www.burnswatergardens.ca/bwgiris.html

Your USDA Zone 3 climate will be perfect for them.

Others to consider:

Physostegia is VERY cold and moisture tolerant.  Will it endure the boiling Summer temps you expect?  Maybe.  If you plant this one, expect to use a lot of them -- it looks best in large drifts that blend the color together (and make a dense screen to block view of things you don't want to see).

Aruncus dioicus or Aruncus sinensis - 'Goatsbeard', an ugly name for a graceful shade-lover.  It takes at least two seasons to become established; after that, you'd think it had grown there for decades.  Because it does not transplant reliably, however, it may never come around in the conditions you are giving it.  I would try it anyway.

CIMICIFUGA Racemosa - 'Snakeroot'.  Same problems as Goatsbeard.  This is a very nice plant.

Any of the New England Asters.  I grow Alma Potschke and a few others.  Late season bloomers and very aggressive.  Grow in sun or shade.

Hostas may not be tall enough for your needs, but in a foreground, they may be welcome.  Consider the fragrant 'Aphrodite' or species plantagenea.  These disappear when they go dormant for Winter.

Good luck, and keep me posted.

THE LONG ISLAND GARDENER