Attainable standards of accuracy in the retrodiction of palaeodischarge from channel dimensions

The most easily measurable palaeodimension on underfit streams is the wavelength of valley meanders, whether on manifest or on Osage‐type underfits. Checks that use data for existing, streams, however, suggest that retrodiction from wavelength is likely to be less accurate than retrodiction from width, while this in turn is less accurate than retrodiction from cross‐sectional area, or from some combination of cross‐sectional area and slope. Against this, former width and cross‐sectional area can be difficult to determine with any precision. Even where dimensions are known, calculation of discharge from these alone seems likely to range between some 75 and 130 per cent of actual values at the 50 per cent confidence limits, and between some 45 and 225 per cent at the 95 per cent limits. Variation in the form ratio appears to make no contribution to the difference between observed and expected velocities through the cross‐section. Unless former roughness can be reconstructed, by some other means than appeal to the form ratio, or unless something can be done with sediment transport, the retrodiction of palaeodischarge appears likely to remain subject to much uncertainty.