Mystery Mammal

The backyard of my house backs onto a drainage ditch. Today, on the edge of the lawn and the ditch, we saw a curious little mammalian. It was a brownish, grayish dark color with a long, bald tail. My mother thought it was a rat, since they apparently live in the ditch, but it really didn't look like it to me, so I went out to investigate. He was so focused on rooting away in the grass, looking for whatever it is he eats (roots or insects, I guess) that I got right up to him. He didn't even look in my direction. His tail was rather rat-like (though the tail was black and I've only seen flesh-colored rat tails), but it didn't have a ratty head. Its head was almost beaver-like, I thought, and much cuter than your typical rat.

When I got back inside, I reported that it didn't look like a rat to me. Then we thought it might be a possum, since they're the only other mammals with a bald tail we could think of. But I've just spent some time researching on the Internet and possums don't have black tails and they're more grayish than brownish. I can't find any animal, really, that squares with what I saw. The closest were like nutria and woodchucks, but the ditch is dry and we don't live near any ponds or creeks, so they'd have no reason to live there.

Anyone have any ideas as to what it was? We have a colony of rabbits in our neighborhood, but we've never seen any animal like the one today. Should we be worried that we have some sort of pest infestation or is it just a harmless creature rooting for food?