leyland cypress

Cupressus × leylandii

You'll need to determine if this grows well where you are, but hearing that the conditions are wet, the first thing that comes to mind for use as a loose hedge is winterberry holly. The larger the mass, the better the show -- in winter -- when everything else is drab. It looks like you don't need but about 10' of height in order to screen out the undesirable parts of the view (the trees and sky beyond not included.)

Often coarse, looming, sloppy and boringly generic looking Leyland cypress is now so over-planted they passed a law against hedges of it in Britain - its place of origin. In a damp soil its rapidly heavy top is likely to pull the whole thing over during high winds or heavy snowfalls, particularly since production nurseries often fail to keep rapid growing items like this potted on in a timely manner - end consumers will most of the time be planting root-bound examples that will presumably not be able to achieve a complete, normal anchorage.

Oregon coastal native Pacific wax myrtle has done well on a sopping clay bank at the government locks in Seattle. And there is some of this planted at interchanges on the part of I-5 that goes through the Willamette Valley. So there might be some scope for some of this in your screen project.

Western red cedar is climax in local wetlands, might work well as part of your screen. This is of course a giant conifer in time, on suitable sites.

Other evergreen plants that can be used where it is moist are various hardy bamboos including members of Phyllostachys, Pleioblastus, Pseudosasa and Sasa. Bamboos belonging to these genera will however spread underground to form patches on many sites, and will not be likely to grow where surface water sits for very long.

If you are anywhere at all near Portland (I'm drawing a blank on Polk County) The Bamboo Garden nursery is a good place to see these. Otherwise additional Oregon vendors can be found using the online Bamboo Species Source List provided by the American Bamboo Society.

If many deciduous kinds would be acceptable then

Alders

Serviceberries

Birches

Hawthorns

Sweet-bay magnolia*

Medlar

Poplars

Wing-nuts

Pears

Pin oak

Willows

may have some potential for you. These are all trees - sometimes quite big trees - if tall shrubby kinds (such as the aforementioned wax myrtle and bamboos) would be okay then the list of candidates becomes much longer.

In general your main, perhaps severe limitation will be the standing water in winter. Places where this occurs locally where native vegetation is still present are easily spotted due to the occurrence of certain indicator species such as black hawthorn, Oregon crab-apple, red-twig dogwood and willows that can tolerate this condition. If you can instead install drain lines and have an area this water can be drained into then you will be able to choose from a vastly larger range of plants than what is probably available to you now.

*Named evergreen forms are on the market

I'm sure you've figured it out by now, as the date shows that this post is over six years old, but I love things that serve double-duty: color, winter interest, provides food for people and wildlife, etc. I know that elderberries like moist conditions, and I believe you might even get away with some rhododendrons. A number of years back, I cut some flowers to put in a bouquet, and that really helped my bush get dense and "bushy!" Pretty soon, it was taller than the roofline! The leaves are evergreen, so they would not lose their leaves in the cold. A mix of evergreens would be pretty, too. I have some balsam and blue spruce out back that I love...although I'm not sure how they do with excess water, being we have very sandy soil here. Google "plants that thrive in wet conditions zone___, and I'm sure you'll get more than enough to work with.