The tin idea gave me a direction to go. After a bit of fiddling. I got the solution I was looking for. I created a tin for the river that only included that area. I created another tin for the rest of the area that used the exact verticies of the river boundary. Then I used a 2d scatter file to create a tin for each layer using the appropriate base tin (river or surrounding) putting them into an appropriately named folder. Using horizons to solids, I used the tin folders and put them to the river or surrounding tin. I then combined the solids that were created into a single folder. That created a perfectly vertical contact.