MORPHOLOGIC CHARACTERISTICS OF STEP POOL STREAMS ALONG THE SOUTHERN SLOPES OF GRAND MESA, COLORADO

Step pool streams are ubiquitous mountain features.  Broad, elongated valleys and distinct fluvial step pool sequences were created as a result of runoff from a late Pleistocene ice cap that modified valleys extending from Grand Mesa, Colorado.  Today, these streams are characterized by steep channel gradients, with steps composed of igneous clasts and woody debris and pools filled with finer sediments.  Step pool systems of Grand Mesa are unique because they are fed by numerous, regulated, large lakes located on top of the mesa; in contrast to most first-order mountain streams. In addition, controlled discharge information for the past century exists for these lakes. Our objectives were to characterize influences of present-day hillslope processes and post-glacial geologic conditions.

We have mapped the occurrences of three step pool streams and documented their morphology and process of formation. During the last century, Grand Mesa has been exposed to a number of anthropogenic activities, potentially causing impacts on the natural step pool configuration. Step pool length varies from 4 ft to 12 ft. Step height varies from 0.25 ft to 2.0 ft. Step clasts range in size from 0.4 ft to 2.2 ft; pool sediments range from silt to >2 ft.

Analysis of step pool morphology and dominant processes was accomplished through the use of geomorphic mapping using GIS and remotely-sensed data and field data collected during Summer 2009.  Stream characteristics such as channel profile and width, step composition and height, pool spacing and particle size, and dominant hillslope processes have an impact on step pool formation. This research expands current understanding of the controls on step pool formation in mountain settings.