LITHOLOGICAL IMPACTS ON STEP POOLS IN ALPINE STREAMS IN THE SAN JUAN MOUNTAINS, COLORADO: A PRELIMINARY STUDY

Step pools are a series of pools separated by abrupt changes in the longitudinal profile of a stream. A common feature of many streams and rivers that flow across a flood plain is the meandering pattern. Meanders, which occur along a horizontal plane, are a way in which a river dissipates energy. Step pools are very similar to meanders, except they occur in a vertical plane rather than horizontal. Whereas a considerable amount of research has been undertaken to explain the formation of step pools in streams in mountainous regions, almost no research, unfortunately, has focused on whether lithology affects step-pool characteristics. Thus, we have investigated step-pool streams in three different lithologies: sedimentary, metamorphic and igneous; the igneous rocks in this area are primarily volcanic pyroclastics.

The study area is located in the San Juan Mountains of southwestern Colorado. We measured the longitudinal profile and cross-sections, as well as documenting the type of lithology, and channel form. Our sample streams included both step-pool series formed by clasts and step pools in bedrock.

The results from our study suggest that no noteworthy difference appears to occur between a bedrock step-pool series and an in situ step-pool series. Regarding the longitudinal profile of a step-pool sequence, the connection between geometry of the channel, step wavelength of the steps and height of the steps from pool to top of a step is similar to results reported for other studies of high-gradient streams. Generally, lithology does not appear to influence step-pool size, spacing or depth of the pools. Channel gradient, width, and particle size seem to have a greater effect on step-pool morphology than does lithology. Channel width appears to be an additional important variable in influencing step wavelength and step-to-pool change in elevation for these streams.

We suggest that because lithology seems to have a minimal effect on step-pool spacing, energy dissipation and stream gradient are the fundamental physical attributes controlling step-pool spacing.