
River birch is supple and bends a long way under ice rather than snapping, but bending is the failure mode — fine twigs and dense tops load with glaze and clump stems fold to the ground, and many never stand back up. That turns into a reduce-or-remove decision rather than a broken-limb cleanup, and because birch decays fast, trying to salvage a folded stem usually costs more than it returns.
Prune river birch in late summer or fall, or after the leaves have fully expanded in early summer — never late winter or early spring, because it bleeds sap profusely from cuts made when the sap is moving. The clump form is the real trap: the moment to remove a crossing or redundant stem is in the first ten years, since a mature clump stem leaves a wound the tree cannot close and birch decays quickly behind big cuts.