Stream Bank Erosion Inventory on Hangman (Latah) Creek
Erosion and sediment delivery in Hangman (Latah) Creek has been a significant nonpoint source of pollution to the Spokane River for hundreds of years. One of those sediment sources is the easily erodible stream banks. From the loess (wind-blown) soils in the headwaters to the coarse sandy banks in the lower watershed, Hangman (Latah) Creek cuts its’ way back and forth through the valley seeking hydrological equilibrium. Flashy flows and high-energy storm events often result in murky sediment-ladened flows in the winter and spring. The causes of erosion are many and include both natural and human influences. Spokane Conservation District (SCD) began investigating this issue about 20 years ago.
In 2003, Spokane Conservation District’s Water Resources Team conducted its’ first stream bank erosion inventory on the main stem of Hangman (Latah) Creek. Nineteen years later, we completed another inventory to compare and understand changes. This effort was accomplished by floating the creek in canoes over five days. We started at the Idaho state line and floated approximately 56 miles to the confluence with the Spokane River. All actively eroding stream banks were documented and measured. We took hundreds of pictures and noted observations of changes in riparian vegetation communities, floodplain connections, and land uses.
SCD recorded 127 actively eroding banks which totaled 37,369’ linear feet. We also documented an additional 27 banks were slumping (approximately 7,000’). Actively eroding banks ranged in height and length according to location in the watershed. In the upper watershed, banks averaged between 3-10’ in height and 200’ in length, whereas the lower portion of the watershed had banks averaging 15-65’ in height and 420’ in length. Although the watershed has abundant erosion, the SCD documented a 10% reduction of actively eroding banks (approximately 4,000 linear feet) from the 2003 inventory. This was good news to observe and document the environmental improvement. It further illustrated those continued efforts by the SCD, private landowners, and other entities to stabilize banks and restore riparian habitat have had an impact over time. For more information regarding this data, contact Walt Edelen at (509) 535-7274 ext. 224.