A full range of natural flows is needed to maintain the ecological processes of a stream system. Flow variation helps create and maintain the range of habitats necessary for diverse, healthy aquatic communities and ecosystems. For example, pulses of high flows are essential during fish spawning season. The streamflow alteration indicator compares the annual variations in flow hydrographs for the SuAsCo rivers against the yearly variations of a relatively unaltered river in the same region (Squannacook River). It compares the magnitude, frequency, duration, timing, and rate of change of flows. The final score is calculated as the average alteration as compared with the Squannacook for nine key flow statistics.