FLOOD DEPENDENCY OF COTTONWOOD ESTABLISHMENT ALONG THE MISSOURI RIVER, MONTANA, USA

Flow variability plays a central role in structuring the physical envi ronment of riverine ecosystems. However, natural variability in flows along many rivers has been modified by water management activities. We quantified the relationship between flow and establishment of the dom inant tree (plains cottonwood, Populus deltoides subsp. monilifera) al ong one of the least hydrologically altered alluvial reaches of the Mi ssouri River: Coal Banks Landing to Landusky, Montana. Our purpose was to refine our understanding of how local fluvial geomorphic processes condition the relationship between flow regime and cottonwood recruit ment. We determined date and elevation of tree establishment and relat ed this information to historical peak stage and discharge over a 112- yr hydrologic record. Of the excavated trees, 72% were established in the year of a flow >1400 m(3)/s (recurrence interval of 9.3 yr) or in the following 2 yr. Flows of this magnitude or greater create the nece ssary bare, moist establishment sites at an elevation high enough to a llow cottonwoods to survive subsequent floods and ice jams. Almost all cottonwoods that have survived the most recent flood (1978) were esta blished >1.2 m above the lower limit of perennial vegetation (active c hannel shelf). Most younger individuals were established between 0 and 1.2 m, and are unlikely to survive over the long term. Protection of riparian cottonwood forest along this National Wild and Scenic section of the Missouri River depends upon maintaining the historical magnitu de, frequency, and duration of floods >1400 m(3)/s. Here, a relatively narrow valley constrains lateral channel movement that could otherwis e facilitate cottonwood recruitment at lower flows. Effective manageme nt of flows to promote or maintain cottonwood recruitment requires an understanding of locally dominant fluvial geomorphic processes.