Good Morning Jill, 

Thank you for contacting the U of M Extension Service. 

I agree the missing bark does appear concerning. Several factors to consider: was the tree watered during the drought? Has there been any other stressors for the tree? It almost looks like a falling branch or something may have scraped the bark off. 

I would attempt to inspect the area up close for potential woodpecker damage. If there is a pest/insect infestation, the woodpeckers would pick the bark off searching for the insects. Is the canopy leafing out? 

Here is a link about river birch from the Morton Arboretum: https://mortonarb.org/plant-and-protect/trees-and-plants/river-birch/

I cannot identify any specific problem from your photo. If the bark is still intact around the balance of the tree, it should survive. I would closely monitor how the tree performs this summer. It appears to be a high value tree in your yard. You might consider hiring a certified arborist: https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Good Luck!

You are welcome!