Quantifying Channel Change with Historical Maps and High-Resolution Topographic Data: Lower Feather River, CA, 1909-1999

Channel and sediment storage changes over a 90-year period in a large alluvial river were quantitatively measured at two sites using standard remote sensing and other GIScience methods. The magnitude of change was measured in a river adjusting to severe human impacts, including deep aggradation by hydraulic gold-mining sediment, flood-control levees, and channelization. High-resolution 1909 historic topographic maps (1:9600 with 0.6-m contour intervals) were converted to 3x3-m digital elevation models (DEMs) by digitizing contour lines and other topographic data. The 1909 DEMs were vertically co-registered with 3x3-m 1999 LiDAR/SONAR DEMs and subtracted from them to compute a 3x3-m rasterized elevation-change map. These grids were partitioned into channel, bar, and terrace areas and used to map volumetric change over the period.

The resulting maps of contemporary topography and elevation change reveal evidence of several geomorphic processes. Natural levee development is strong at both sites on the 1909 maps—even where a channel diversion had occurred only a few years earlier—but much weaker on the 1999 maps. Net erosion predominated at both sites, but was partially negated at Shanghai Bend by filling of an abandoned channel. Erosion at both sites was concentrated in low-water channels and bars, while higher surfaces (terrace treads and channel fill) experienced net deposition. The combination of channel erosion and terrace deposition resulted in greater floodway relief and lower lateral connectivity between channels and overflow areas.

A series of aerial photographs from 1952 to 1999 was rectified and channel positions were digitized. Comparisons of lateral channel migration rates before and after 1952 at the two sites indicate that channel morphologic changes and sediment mobility were much greater in the first half of the 20th century than in the second half.