In regulated rivers, the relationship between wetted perimeter and discharge is sometimes used as an expedient technique for determining the minimum flow allowable for environmental purposes. The critical minimum discharge is supposed to correspond to the point where there is a break in the shape of the curve (usually a logarithmic or power function). Below this discharge, wetted perimeter declines rapidly. This critical point on the curve is almost universally, but incorrectly, termed an 'inflection' point, and is usually determined subjectively by eye from a graph. The appearance of a break in the shape of the curve is strongly dependent on the relative scaling of the axes of the graph. This subjectivity can be overcome by defining the break in shape using mathematical techniques. The important break in the shape of the curve can be systematically defined by the point where the slope equals 1, or where the curvature is maximized. The technique can be applied to other habitat -discharge relationships, provided the habitat variable increases with discharge. These techniques were applied to two regulated headwater streams located in the Melbourne catchment area. Channel survey data were used to model the relationship between discharge and wetted perimeter, flowing water perimeter and blackfish habitat area. A logarithmic function could be fitted to the wetted perimeter data for Starvation Creek, but the relationship for Armstrong Creek was linear. Both streams showed logarithmic relationships between discharge and flowing water perimeter. For these streams, the wetted perimeter relationships did not suggest an optimum environmental flow, nor did they suggest a flow level that would maintain the macroinvertebrate community in its unregulated state if it was applied for a long period of time. Fish habitat area does not necessarily increase with discharge, so the method of curve analysis suggested here for wetted perimeter may not be applicable to some fish habitat area data. Flowing water perimeter is preferable over wetted perimeter as a variable to define habitat suitable for macroinvertebrates.