I love alder and small willows, personally.

Josh

I have similar area on my property although in spots it dries out by mid-summer. I have experience with and would recommend the following plants-all are native North America:
Trees (northeast/southeast portion):
Acer rubrum (swamp maple)- acid to slightly acid soil
Betula nigra - river birch - acid to slightly acid soil-there also a cultivar that is shrub size 'Little King'
Carpinus caroliniana (american hornbeam)-neutral to slightly acid soil
Querus bicolor (swamp white oak)-neutral soil
Querus palustsris (pin oak) - acid soil
Taxodium distichum (bald cypress)-acid soil

Shrubs:
aronia arbutifolia (red chokeberry)
aronia melanocarpa (black chokeberry)
Cephalanthus occidentalis (buttonbush)
Clethra alnifolia (summersweet)-acid soil
Hamamelis vernalis (vernal witchhazel)- slightly acid to acid soil
Ilex glabra - (inkberry holly)-acid soil - evergreen
Ilex verticliata (winterberry holly)-slightly acid to acid soil
Itea virginicua (virginia sweetspire)
Myrica pensylvanica (northern bayberry)
Physocarpus opulifolius (common ninebark)-
Viburnum tribolum -(american cranberrybush)

Good Luck