Canyon riparian zone vegetation is vulnerable to effects of upstream river regulation. We studied box elder (Acer negundo) dominated canyon riparian forests intensively on the Green and Yampa rivers in Dinosaur National Monument, Colorado, and extensively in four other major rivers of the upper Colorado River Basin to determine the effects of river regulation on riparian tree establishment patterns. We: 1) aged individuals to determine if establishment was correlated with high annual peak flows, 2) mapped cohorts to determine if the areal extent of post-regulation cohorts was reduced on regulated compared to unregulated river reaches, and 3) measured the floodplain position of cohorts in regulated and unregulated rivers to determine if establishment was confined to lower landscape positions under regulated flow regimes. Box elder establishment was highly correlated with annual peak flows, with most recruitment occurring during years with unusually high peak flows. In regulated river reaches recruitment was facilitated by annual peak flows that were below average under a natural flow regime but were unusually high under the post-dam flow regime. The areal extent of post-regulation box elder cohorts was reduced on the regulated river compared to pre-regulation cohorts on all rivers, and recent cohorts on an unregulated river. Post-regulation cohorts on regulated rivers established at lower landscape positions than cohorts on unregulated rivers, resulting in inset floodplain forests on regulated rivers. The reduction in establishment height above the river was directly proportional to the magnitude of post-regulation peak flow reduction. Controlled high magnitude flood releases would facilitate forest regeneration across the full extent of historic forests, and peak flows that would mimic a lower magnitude natural hydrograph would facilitate establishment in the inset floodplain. In an era of increasing consumption of shrinking water supplies, opportunities for high magnitude reservoir releases are likely to diminish, increasing the need for active management of riparian forest ecosystems.