Vegetation patterns along the Middle Fork of the Flathead River, Glacier National Park, Montana

1992 Spring.Includes bibliographic references (pages 95-103).Distribution of riparian vegetation on riverine floodplains is dictated, in part, by species' response to flood disturbance. Plant position can be related to frequency, intensity and duration of flooding, with species most tolerant of flooding occurring in lower discharge classes and those less tolerant occurring in higher discharge classes. Hydrologic conditions necessary to support distinct vegetation cover types and key indicator species along the Middle Fork of the Flathead River were identified and quantified. The natural flow regime of the Middle Fork was described in terms of the magnitude, frequency, and duration of discharge. Vegetation patterns were determined from species presence and relative abundance observed in transects located perpendicularly to the river and extending across the valley floor. A hydraulic model was used to compute the minimum discharge required to inundate each point along the transects. TWINSPAN (Hill 1979) analysis was used to identify four distinct cover types and several key indicator species. Direct gradient analysis was used to array cover types and key indicator species along a hydrologic gradient of inundation duration. Plots within the unvegetated or barren cover type were characterized by a minimum inundating discharge of 250 cubic feet per second (cfs) and a fraction of time inundated of 10 percent or greater. The willow cover type was defined by flows between 1000 and 15000 cfs (which correspond to recurrence intervals of < 1 - 2 years) and a fraction of time inundated of 0.45 to 25 percent. The cottonwood cover type was distinguished by flows ranging from 10 to 25 thousand cfs, a recurrence interval of 1 to 10 years, and a fraction of time inundated of 0.25 to 2 percent. Plots within the spruce cover type were characterized by flows greater than 25000 cfs, a recurrence interval of 10 years or more, and a fraction of time inundated of less than 0.35 percent. Key indicator species within a cover type (described in terms of density and percent absolute cover) were characterized by distinct hydrologic conditions representative of that particular cover type