Riverine floodwater pulses provide water, nutrients, and sediments to floodplain wetlands, but flood pulses also act as a natural disturbance by removing biomass, scouring sediments, and delivering turbid waters. We investigated nearly pristine montane floodplain wetlands with varying degrees of river connectivity in Jasper National Park, Canada: three fully connected riverine marshes, three partially connected beaver-impounded marshes, and three completely disconnected railway-impounded marshes. Our objectives were to determine how river connectivity affects plant biomass and water and sediment chemistry, estimate impacts of nutrient limitations on plant biomass, and compare natural variations in river flooding on plant biomass and nutrients. The amplitude of water level fluctuations, a measure of flood disturbance and river connectivity, was highest in flood years and in riverine marshes. Sites with river connectivity had significantly higher plant biomass than sites without river connectivity (railway-impounded marshes). NO₃⁻-N, TP, and turbidity correlated positively with river floodwater pulses. A high flood year increased nutrient supply in the wetland water but significantly decreased plant biomass in all sites. Moderate flood disturbance and nutrient inputs from floodwaters provided optimal growing conditions for plants in these montane floodplain marshes.