PHYSICAL AND WATER CHEMISTRY CHARACTERISTICS OF MOUNTIAN STREAM STEP POOLS; SAN JUAN MOUNTAINS, CO

Step-and-pool series in steep mountain streams have captured the attention of numerous researchers over the years. The morphology of these features has been investigated at various locations, but unfortunately, water chemistry associated with step-and-pool series has not been addressed. Our study uses longitudinal and cross-section surveys of several streams in the San Juan Mountains to describe channel form, to compare water chemistry with channel form and to develop an object-oriented dynamic model to simulate change in these streams. We measured both step-pool series in bedrock and step-pool series formed by in situ clasts.

Channel gradient and width, dimensions of the step-pools, type of lithology, and number of clasts forming the steps or number of particles in the pools were surveyed in the field. Water chemistry was measured in the field using a YSI^® Professional Plus multi-probe to measure temperature, pH, specific conductance, and dissolved oxygen.

The results from our study suggest that no significant difference was found between bedrock step-pool series and in situ step-pool series. In addition, channel gradient, width, and particle size have various effects on step-pool morphology. With respect to the longitudinal profile of a step-pool sequence, the relationship between channel geometry, step wavelength and step-to-pool change in elevation is similar to results reported for other studies of high-gradient streams. The channel width and particle size appear to be important variables affecting the step wavelength and step-to-pool change in elevation for our streams. Whereas the width of a step appears to be regulated by particle size and width of the channel, pool geometry appears to be related to channel gradient, discharge, and particle size. These characteristics also appear to influence difference in temperature, specific conductance and dissolved oxygen between the water flowing over a step and water in a pool. There appears to be no significant difference in pH between water flowing over a step and water in the pool. We suggest that the geometry of the step-pool can be explained by particle size, discharge, and channel gradient and a difference in water chemistry between steps and pools occurs.