Landscape level historical analysis of braided river braid plain composition, age, and formation, Matanuska River, Alaska - Monica McTeague , USGS (co-authors: Janet Curran/USGS)

ABSTRACT

A historical perspective on braid plain activity in the Matanuska River, Alaska from 1949 to 2006 provides an opportunity to explore braid plain dynamism at the landscape level. The glacial Matanuska River spans more than 75 miles and is braided over 85 percent of its course. Delineation of braid plain features on the basis of patterns of vegetation in historical aerial photography provided data for analysis of braid plain composition and age. The historical braid plain was mapped from 1949, 1962, and 2006 aerial orthophotography into categories including active channels, unvegetated bars, and vegetated surfaces with evidence of channel occupation. Further categorization of vegetated surfaces included assessment of position as mid-channel (island) or attached to the valley margins and age as 0 to 44 years, 44 to 57 years, 57 to about 100 years, and more than 100 years. In 2006, about 40 percent of the Matanuska River braid plain consisted of vegetated surfaces; of these about 75 percent consisted of surfaces attached to the valley margins that were vegetated 0 to 44 years ago. Only about 10 percent of the vegetated braid plain features in 2006 were islands. Few vegetated surfaces persisted over the 57 year time period, with the majority of these (about 10 percent of the braid plain features) being surfaces attached to the channel margins. The average age of the braid plain was estimated at 9 to 35 years from the areas and estimated ages of the categories of vegetated surfaces and unvegetated bars. Although braid plain turnover rate is high over the historical period, the relative distribution of surfaces attached to the valley margins and in the middle of the channel remained fairly consistent between years as did the relative distribution between vegetated and unvegetated surfaces. Historical erosion of the braid plain margin, including terrace and fan features, was much more localized whereas broad-sweeping changes occurred within the braid plain. The formation of the young, mainly unvegetated braid plain provides essential information to understand historical river process and to document change over time. Topic: Other

Topic: Other