Large floods in Alaska: Considerations of Flood-Generating Mechanisms and Contributing Factors

Author: Curran, Janet H., USGS Alaska Science Center

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Abstract

Discharge-related floods in Alaska can be attributed to a range of flood-generating mechanisms. Large floods associated with spring snowmelt have occurred in relatively cold, dry areas. Elsewhere, rainfall, snow-and-ice melt from high elevation or glacierized basins, a combination of rainfall and melt, and glacier lake outburst floods (GLOFs) have generated large floods. Although some of these flood populations can be considered similar for analyses like determining flood frequency and magnitude, some form statistically distinct populations. Identifying these populations can inform flood frequency analysis and improve the understanding of the effects of climate change on floods when coupled with understanding of projected changes in the mechanisms.

Several recent floods in Alaska demonstrate factors contributing to or attenuating large floods. In one case, record fall precipitation during a cool period generated flooding in lower-elevation, smaller streams but elevational gradients in the rain/snow fraction ameliorated runoff production at higher elevations and limited flooding in larger streams. In contrast, heavy fall rainfall coinciding with warm temperatures generated an atypically late large flood in a large stream. In a third example, intense rainfall generated record flooding from likely convective storms, less commonly documented as large-flood-generators in Alaska than atmospheric rivers. These floods help identify the role of variables like temperature, elevation, and atmospheric condition needed to address the complexity of flood-generation from rainfall.

Glacier lake outbursts have created some of the largest gaged floods in Alaska, floods that are outsized relative to non-outbursts on the same stream. Other gaged GLOFs have a distribution ranging from larger than to close to non-outburst floods, and a life-cycle of GLOF magnitude and seasonality can be seen for some glacier lakes. An in-progress inventory of gaged GLOFs will improve accessibility to streamflow data for this flood population.

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