HYDROGEOLOGY OF THE BOULDER RIVER WATERSHED STUDY AREA, MONTANA, AND EXAMINATION OF REGIONAL GROUND-WATER FLOW USING INTERPRETED FRACTURE MAPPING FROM REMOTE SENSING DATA

This investigation describes the shallow unconsolidated and upper fracture controlled ground-water flow regimes in the Boulder River watershed, and uses remote sensing data to map linear surface features that are interpreted as fractures and are assumed to be associated with regional fracture controlled sub-surface flow. The hydraulic properties of the bedrock aquifer, at depth, are largely unknown. However, along fault zones, mineralized zones, and some large fractures, the hydraulic conductivity may be significant. A conceptual model of ground-water flow was developed based on hydrogeologic characteristics found througout the watershed. The direction of shallow ground-water flow can be modeled as the gradient of topographic slope. Deeper ground-water flow likely is a function of fracture width, continuity, and interconnectivity, and likely is partly controlled by the heterogeneous and anisotropic orientation of fractures. Four remote sensing data sets used as base images, provided a range of spatial resolutions for linear-feature mapping. Endpoint data from mapped linear features, used in a 2-D orientation analysis, enabled us to determine primary orientations of lineaments from rose diagrams and to produce contour maps of linear-feature characteristics. A contour map of combined linear feature spatial frequency and intersections was compared with a map of wetland-soils areas to identify correlated areas of high fracture occurrence and areas where the potentiometric surface is close to the land surface. The mapped lineaments show a consistent east-west primary orientation. The greatest linear-feature spatial frequency, length, and frequency of intersections occurred on the Continental Divide at the northern boundary of the Boulder River watershed study area. The observation of highest interpreted fracture intensity on the continental divide is significant in that this area may provide potential recharge for fracture-controlled ground water. The comparison of highest interpreted fracture frequency with areas of mapped shallow ground water shows a correlation in several locations between potential areas of recharge to the fractured granitic aquifer and areas of discharge, or wet soils.