You might want to check with your local native plant society (which I think might be quite active: we visited Chicago this summer and I was amazed at all the native plants used in the municipal plantings)for suggestions of plants that are native to your area and would work in the former buckthorn heaven. My first thought for bank stabilization was willows, and every area has different native willows, of varying sizes.
Regarding garlic mustard - pull it as soon as you see it, and don't let it go to seed. RoundUp will be your friend if you've got lots of it. You could also mulch the area before anything sprouts at all, which may keep somethings from showing up at all.
Good luck!

How about planting an assortment of sedges to take over the area. Many sedges can tolerate varying amounts of shade and moisture. Look for sedges that can also protect your bank from erosion.

Garlic mustard is a biannual. First year plants forms a rosette close to the ground and it is not likely to flower. Second year plants gets much taller and will flower and set seed. After releasing seeds in the second year, GM dies. The problem is that GM reseeds prolifically and seeds can lay dormant in the soil 7 years or more. Pull plants as often as possible before plants begin to flower. Remember that once GM flowers, the task becomes more difficult because GM will need to be bagged and removed from the site. The downside of pulling is that pulling will expose more seeds to sunlight, encouraging more of the garlic mustard seed-bank to germinate. Spraying with herbacide is another effective method. Spray herbicide in very-early spring because GM is one of the first plants to emerge in late-winter, allowing you to isolate and target the GM better. Weed-whacking is another effective solution. This is best accomplished when plants are freshly in flower, but before seeds are forming. Weed-whack plants as close to the ground as possible. If you leave too much of the plant still standing it might have enough energy to set a second round of flowers. You can correct this by repeatedly weed-whacking if necessary.--better to get right to the ground the first time. Remember, you'll not likely eliminate the problem in one year but with each passing year you'll see progress.

Sounds like a worthwhile project, Chisue. One thing to be aware of is the high likelihood that there are numerous buckthorn seeds lying in waiting to sprout. We've cleared hillsides of this stuff, treated the cut surface with appropriate herbicides, etc. Now, ten or more years later, those sites are sadly completely covered with buckthorn again. It can be quite a battle.

In addition to the suggestions above, you may be able to employ rhizomatous fern species in this project. Such species as ostrich fern, sensitive fern, and cinnamon fern, can do battle with many other plants due to their ability to form large colonies. The first two species I mention will also tolerate a surprising amount of sunlight if moisture is plentiful.

+oM

Congrats on removing that Buckthorn!! What a scourge it is. Over the past 5 years, I've cut the large specimens on my lot and applied a concentrate brush killer to the cut stumps. They had made a zillion seeds though, so there are lots of small Buckthorns and some resprouting that occasionally happens. This seems to die quickly using the a light spray of Brush-B-Gone at the recommended solution on the bottle. Since you are near a river, are you using a formulation of herbicide that is safe near water?

Perhaps you could plant some shade tolerant and moisture loving shrubs to replace the Buckthorn. Common elderberry (Sambucus canadensis) or Silky Dogwood (Cornus amomum) might work and both make berries that the birds love. They also sucker, which is good for stabalization. You can buy seedlings that are inexpensive (approx. $1 apiece), and with some moisture loving shrubs you can take cuttings at the right time and bury them directly in the ground with good success.