I think this is probably Macrobdella decora, a large freshwater leech (this one was about 2 inches). I found this one seemingly feeding on wood frog eggs (those black dots) in a large trailside drainage ditch, while wood frog tadpoles swam around it, and water striders, water boatmen, and caddisfly larvae were also abundant. The Macrobdella decora is known to feed on amphibian eggs, although it prefers blood. Once it has fed, it can last up to a year or more without feeding.

Roadside/trailside ditches, ponds permanent and ephemeral, are fascinating places!