The Marias River flows from Glacier National Park through northcentral Montana, and into the Missouri River. Annual flows gradually declined from 1902 to 2024 (similar to 3.2%/decade) and the 1952 Tiber Dam and Lake Elwell reservoir were operated to attenuate peak flows and stabilize downstream flows year-round. We had assessed riparian woodlands along the Upstream versus Downstream Reaches in 1991-1994, and for this study we reassessed the riparian conditions three decades later, in 2025. Following the flow stabilization, the downstream river channel was stabilized, and dense herbaceous vegetation encroached into the Colonization Band between the river and the mature riparian woodland. Consequently, there was sparse seedling colonization of plains and narrowleaf cottonwoods (Populus deltoides and P. angustifolia) along the Downstream Reach, in contrast to prolific colonization along the relatively free flowing Upstream Reach (lineal extent with colonizing cottonwoods: Upstream 82%, Downstream 35%). Conversely, colonization by the native shrub, silver buffaloberry (Shepherdia argentea), and related, non-native Russian olive (Elaeagnus angustifolia), was more extensive along the Downstream Reach (lineal extent: Upstream 1.4%, Downstream 52%). Additionally, the mature cottonwoods appeared healthier Upstream, with less branch and crown dieback. Extending from these post-dam patterns, the riparian cottonwood forest downstream from Tiber Dam will probably decline further without a more naturalized river flow regime, with regular seasonal variation and occasional higher flows to provide geomorphic disturbance, disfavor encroaching vegetation, and enable cottonwood replenishment. Similar to other regulated rivers, any prospective environmental flow regime below Tiber Dam must also consider the other social and environmental outcomes.