Left: Leaf and staminate catkin of Paper Birch
Right: Leaf and staminate catkins of River Birch.

Top Left: Paper Birch (rounded base)
Top Right: Gray Birch (very triangular)
Bottom Left: River Birch (tapered base)
Bottom Right: Sweet Birch (larger leaf, rounded base)

Top Center: Gray Birch
Bottom Left: Paper Birch
Bottom Right: River Birch

Top: River Birch
Bottom: Paper Birch