Residents of the Mat-Su Valley who have watched the braided, glacial Matanuska River eat away at homes, property and roads for decades now have a better understanding of the river's erosion and what the future may bring.
A new report from U.S. Geologic Survey’s Alaska Science Center details which sections of the river are most prone to erosion. When the river flows beside a sand-and-gravel bank, erosion can be swift. For instance, more than 200 feet of riverbank in a hard-hit residential area near the community of Butte washed away from 2004 to 2006.
“Erosion has typically occurred in bursts, rather than as a regular amount over a long time. Knowing how erosion occurs provides a good perspective for addressing future erosion,” said Janet Curran, a USGS hydrologist and one of the study authors.
The study looked at vegetation and channel patterns in historical photos, detailing how much of the Matanuska River’s 74 miles of bank has eroded since 1949. Because the average age of vegetation on the braid plain was estimated to be only 15 years, it was clear that the river moves frequently. Some channels have remained stable for decades, allowing trees to grow, but entire forests were quickly removed when the river re-occupied its former path.
"Rivers can be both capricious and powerful as they plow back and forth across the landscape on their way to the sea, but even they must obey the laws of geology," USGS director Marcia McNutt said in a press release. "This new study maps out locations of unconsolidated banks that rivers can quickly erode and solid bedrock banks that will resist the river's force, identifying safer ground for the long term."
The study is available here.