Frequent Mass Movements From Glacial and Lahar Terraces, Controlled by Both Hillslope Characteristics and Fluvial Erosion, are an Important Sediment Source to Puget Sound Rivers. Issue 4 (30th March 2021)

• Record Type: Journal Article
• Authors: Scott, Daniel N.; Collins, Brian D.
• Is Part Of: Water resources research. Volume 57:Issue 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-30
• Subjects: erosion -- paraglacial -- Puget Sound rivers -- sediment delivery -- terrace; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2020WR028389
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: Mass movements from glacial and lahar terraces in the middle and lower reaches of rivers draining the Washington Cascade Range to Puget Sound may represent a substantial but poorly quantified portion of those rivers' sediment supply and pose significant mass movement hazards. We used repeat LiDAR elevation data, aerial imagery, and well logs to quantify and characterize terrace sediment delivery in nine major watersheds over a median period of 12 years. In the 1, 946 river kilometers for which repeat LiDAR was available (71% of the 2, 736 total river kilometers flanked by terraces), 167 mass movements eroded 853, 600 ± 19, 400 m 3 /yr. Analysis of mass movement frequency and volume indicates that terrace sediment delivery is dominated by small, frequent mass movements, as opposed to large, infrequent ones like the 2014 Oso landslide. This sediment source is low in river networks, well connected to streams, and has a substantial coarse‐grained and durable component, all of which increase its significance to sedimentation in developed, lowland reaches. However, rates of terrace sediment delivery vary among basins and between adjacent terraces, which are stratigraphically laterally heterogeneous. While lateral fluvial erosion is usually necessary to initiate terrace mass movements, valley bottom geometry and terrace stratigraphy poorly predict erosion volume, which is better predicted by hillslope geometry and mass movement style. Effective management of sedimentation and mass movement hazard should acknowledge the importance of terrace sediment delivery and the variability among and within watersheds in sediment delivery, sediment characteristics, and failure mechanisms. Plain Language Summary: Extensive terraces, composed of sediments derived from past glaciations and volcanic mudflows, flank rivers in the Puget Sound region. Landsliding of these terraces creates local hazards but also delivers sediment to rivers which transport it downstream to developed, lowland areas, where sediment can build up and cause river migration and flooding hazards. Measurements of the land surface through time show that these terraces deliver large volumes of sediment to rivers that may have a significant effect on rivers downstream. In addition, small, frequent landslides deliver more sediment over time to rivers than large, infrequent ones. Examining how both river erosion and the composition and shape of these terraces influence the volume of sediment delivered by landslides to rivers shows that both factors contribute to determining landslide location and size. Effectively managing landslide and river hazards in the Puget Sound region requires a detailed understanding of terrace characteristics and the erosional patterns of their adjacent rivers. Key Points: Glacial and lahar terraces deliver sediment to Puget Sound Rivers primarily via frequent, small mass movements Terrace sediment has a substantial coarse‐grained fraction, is likely resistant to attrition, and is delivered low in river networks Considerable spatial variation in size and frequency of mass movements is partly explained by hillslope geometry and failure mechanism