Plains cottonwood recruitment and survival on a prairie meandering river floodplain, Milk River, southern Alberta and northern Montana

Abstract

Age, distribution, and density of two populations of plains cottonwood (Populus deltoides var. occidentalis Rydb.) on the floodplain of the Milk River, southern Alberta and northern Montana, were studied in relation to historical river hydrology and sedimentation regimes. In Alberta, cottonwood recruitment leading to long-term survival on river meander lobes (point bars) correlates with years when daily maximum flows during the period of seed dispersal (June 1 to July 10) attain a stage equal to or greater than the 2-year return flood, based on the annual flood series. Such flood events during the seed dispersal period recur an average of once in 5 years. In Montana, on the floodplain for 25 km downstream of Fresno Dam, built in 1939, the densities of cottonwoods recruited since 1939 are significantly lower than on floodplain sites upstream, in Alberta. Results suggest that this is due to a marked reduction in flood magnitude and frequency, rates of sedimentation and meander migration. Based on this study, the prospects for cottonwood survival on floodplains downriver from dams in this and other prairie river valleys are not encouraging unless management measures are taken to reverse the trend.