Using macroinvertebrates as indicators of stream health, random biosurveys of streams are implemented in each BMU (basin management unit) to project the level of aquatic life use support throughout the commonwealth.

This program uses macroinvertebrates (insects and other invertebrates—organisms that can typically be seen without magnification) as the indicator community to determine the overall health of streams in the commonwealth. A randomly designed monitoring network of wadeable streams in each of the five BMUs is employed to project the overall level of aquatic life use attainment in each BMU.

The probabilistic biosurvey is designed on a per basin unit, with criteria provided to make a random, statistically valid draw of potential target streams to sample that will reflect the basin as a whole. Kentucky defines the potential target stream population as wadeable, first through fifth order streams. The target population is drawn from the U.S. Environmental Protection Agency (EPA) River Reach File 3. (The EPA's River Reach File is a geographic and hydrographic database of the surface waters of the continental United States and Hawaii. The structure and content of the Reach File database were created expressly to establish hydrologic ordering, to perform hydrologic navigation for modeling applications, and to provide a unique identifier for each surface water feature.) 

The probabilistic approach to assessing the overall health of streams is used for a number of reasons:

• It provides data that can be statistically applied to all waters in a basin of similar type for assessing the condition of the commonwealth’s aquatic resources.
• It is inherently unbiased and provides a representative assessment of the actual conditions of water quality in a basin.
• It is the only means by which the thousands of stream miles in each basin can be assessed.
• It provides comparable data so trends of the biological health of each basin can be tracked temporally. 

Once each segment is analyzed for level of use support (full support, partial support or nonsupport), calculations are made based on similar streams in the basin. For example, the results of first order streams in the probabilistic assessment are extrapolated to total number of miles of first order streams in the management unit. Nothing can be said about level of aquatic life use support in streams greater than fifth order since these waters are generally not wadeable, and there are no criteria currently available to determine the biological condition of these large rivers. These biosurveys are also used to make specific aquatic life use attainment assessments in the monitored stream reaches. Those assessment results are then used in the 305(b) report.