WATERSHED DELINEATION IN KARST AREAS OF THE TONGASS NATIONAL FOREST, SOUTHEASTERN ALASKA

The geology and climate of Southeast Alaska are particularly favorable for karst development. Extensive areas of very pure carbonate, approximately 2950 square kilometers (km˛) (555,770 acres, 869 mi˛), are found within the boundaries of the Tongass National Forest. Karst systems have developed to one extent or another within all carbonate outcrops. Karst development includes losing streams, sinkholes, caves, epikarst grikes, and resurgence streams. Many of the resurgence streams maintain productive salmon populations. Identifying the source areas of the resurgence streams is crucial to protecting water quality.

Historically, watershed assessments have only considered the topographic boundaries to define various watersheds. This method is difficult in karst areas where surface waters are rare. Four different projects on the Thorne Bay Ranger District have utilized tracer dye studies to illustrate the complexity of these systems and to help redefine watershed boundaries. Through dye trace tests we are able to identify subsurface flow pathways, minimum groundwater flow velocities, and downstream effects, and we are better able to assess cumulative effects on a watershed scale.

Success of groundwater tracing efforts hinges on locating the springs that feed coastal streams. This requires careful inventory of shorelines and margins of the carbonate outcrops. Activated charcoal packets are anchored within these streams and springs to absorb dye. Tracer dye injection points are generally discrete karst features where surface waters enter the systems. The packets are recovered at intervals to allow time for the dye to move through the systems. Tracking the location of dye injection and recovery identifies specific subsurface water flow pathways and gives us minimum flow velocities. Identification of these subsurface pathways enables us to delineate karst watershed boundaries and to understand the potential downstream effects of proposed management.