Channel morphometry is concerned with the definition, measurement and analysis of quantitative indices describing the cross section, the bedform and long profile, and the plan geometry of rivers. Field measurement is usually necessary, although the widths of large rivers, long-profile gradients and planform properties can all be derived from topographic maps and air photographs. Reaches are samples for measurement in various ways. Since downstream changes in channel geometry are step functions at tributary junctions (Roy & Woldenberg 1986), a random sample of junctions may be adopted as long as measured sections are beyond the immediate hydraulic effects of the mixing of flow at the confluence. Alternatively, a stratified random sample within the drainage network of link midpoints (reaches) having predefined stream magnitudes or orders (see Sec. 2.6) will give a representative range of channel sizes.