The BACI approach is a classic method for measuring the potential impact of a discharge, disturbance, or event on the fish and invertebrates of a stream. Such affects can be analyzed by measuring conditions before a planned activity and then comparing the findings to those conditions measured after—an approach that is applicable for comparing the affects of anticipated future activities. If it is an existing activity, it may not be possible to measure the before conditions. In these cases, studies often make use of an unaffected or control stream reach and use those data for comparisons to an affected or impacted reach.