I kept trying to cross-check it but I kept forgetting the name he used, thinking he said 'midgeon.' Finally I was able to get confirmation that it was widgeon and just look a moment to look them up. I can see the similarities but it's not the same bird I captured. I found this picture on The Bird Zoo. Click on the picture to visit the duck section- it's worth scrolling through the whole list- there are some really great pictures in there.
Another recent development in this mystery bird identification was the latest issue of the Ducks Unlimited Newsletter. The 'Fowl Fact' of the month happened to be about the Green-winged Teal- the only species of duck known to scratch in flight. This picture was too close to question- I compared the coloring on the feathers and it was an exact match. Click on the first image (from Ducks Unlimited) to visit the Green-winged Teal profile page.
Now every time I return to the river I look for them in that same spot, but have yet to see another. I learned this about their preferred habitat from the Ducks Unlimited site:
Tidal creeks and freshwater marshes associated with estuaries are favored over more saline or open-water habitats.
It makes sense then to have seen them in that slow-moving, protected part of the river. I'm really looking forward to seeing what springtime birds will arrive to the river.