Evidence of Paleofloods from LIDAR Imagery of the Matanuska River, Alaska - Janet Curan , USGS (co-authors: none)

ABSTRACT

High resolution topographic imagery derived from Light Detection and Ranging (LIDAR) data provides unprecedented views of land surface texture beneath vegetation and has revolutionized studies of faults and other geologic features. Bare earth surface textures and elevation data for a site near the Old Glenn Highway bridge in Palmer, Alaska, and a nearby site at Falk Lake suggest that LIDAR data may also be useful for documenting paleoflood surfaces along the Matanuska River.

At the two sites, surfaces interpreted as fluvially-stripped bedrock appear in LIDAR hillshade images as rubbly-textured, fin-shaped to irregular surfaces. A scarp rising 2 to 7 m separates these surfaces from the adjacent, relatively smooth, glacial outwash or fan surfaces. Channels are present within the Falk Lake surface. Trimlines, or smoothly arcing scarps at a consistent elevation, are visible along a fan on the left bank at the highway bridge site. Field observations indicate that bedrock is exposed near river level and intermittently within these forested surfaces. At the Falk Lake site, a narrow channel drops in elevation from a high point of 39.6 m near the Matanuska River southward to a low of 30.8 m at a small lake north of Falk Lake, suggesting flow was from the Matanuska River. The nearest Matanuska River bars are 37.0 m in elevation, requiring flows 2.6 m above the modern bars to begin spilling toward the Falk Lake complex. Clear evidence of flows exiting Falk Lake, or entering from the east, are beyond the extent of the LIDAR imagery. At the Old Glenn Highway bridge site, the minimum elevation for overtopping the rough-textured surface is 65 m, or 7 m above the present bars. The stripped surfaces extend up to an elevation of 70 m on the left bank and as high as 73 m on knobs on the right bank. The toe of a second scarp on the left bank is 76 m in elevation, indicating a previous and possibly higher elevation flow. At both sites, the surfaces appear to have been stripped of overburden but not worn smooth, suggesting formation by a large, short-duration flood rather than a previous riverbed elevation. The largest recorded Matanuska River flood, an 82,100 cubic feet per second flow in 1971 caused by a regional rainstorm augmented by a lake outburst in Granite Creek, sent floodwaters through a narrow bedrock gap along the Old Glenn Highway, then westward into the Falk Lake complex and toward the Knik River. However, the flood did not overtop the highway bridge and did not produce stripped surfaces. The incision of the proposed paleoflood surfaces into glacial outwash terraces along the modern Matanuska River valley indicates that their formative flooding occurred after the formation of the major glacial outwash terraces near Palmer. Planned 2011 LIDAR data acquisition with a broader spatial extent may help refine questions and may contribute additional previously undocumented flood evidence.

Topic: Other

Topic: Other