DETERMINING THE SOURCE OF ANOMALOUS SEGMENTS IN A KARST STREAM IN THE DRIFTLESS AREA OF SOUTHEAST MINNESOTA

The Driftless Area of southeastern Minnesota is an area with an extensive karst network. In 2008, Woodside found that longitudinal stream profiles in karst areas show unique anomalies near the entrances to caves. These anomalies are characterized by a drop in elevation where water is being rerouted underground and are followed by an increase in elevation where there is less water eroding the stream bed. Since these overall concave up anomalies are located at or near cave entrances, it is possible that they can be used to identify karst features and cave entrances simply by looking at a stream’s profile. The objective of this study is to determine if these anomalies found in profile are indicative of karst streams and if they actually represent karst features. The two primary methods of generating the stream profiles include using ArcGIS10’s 3D Analyst Tool and surveying the streams in the field with a total station and GPS unit. To assess if the anomalies are diagnostic feature of karst systems, profiles were made on both limestone and sandstone reaches of streams. These anomalies are significantly more prevalent in limestone reaches as opposed to sandstone reaches. The stream profiles were derived from both a 1- and 3-meter DEM. This allowed the different resolution DEMs to be compared, which assess whether a 3-meter DEM can identify anomalies as effectively as a 1-meter DEM. While the 1-meter DEM shows more detail, the 3-meter DEM shows the anomalies and overall shape. For each DEM, profiles were made using an unfilled and filled DEM. The filled DEM filled in sinks and potential errors, and appears to make the karst feature anomaly appear as a step. The anomaly is horizontal and then drops in elevation rapidly. Hack’s Stream Gradient Index was used to calculate a gradient index (SL) for different reaches of streams. The gradient index, measured as gradient-meters, was used to measure reaches of streams with the anomalies and was compared to reaches of the stream with no apparent anomalies. Typically, normal reaches of streams had lower SL values when compared to anomalous regions. The sandstone reaches of streams also tend to have lower SL values, however the values are not as consistent.