Development of an automated desktop procedure for defining macro-reaches for river longitudinal profiles

Abstract

This paper presents an automated desktop procedure for delineating river longitudinal profiles into macro-reaches for use in Ecological Reserve assessments and to aid freshwater ecosystem conservation planning. The procedure was developed for use where there are limited data and/or where a repeatable, statistically defensible regional or national assessment is required. The delineation of longitudinal profiles into macro-reaches between 'controls' or 'break points' such as exposed resistant rock formations, knick points, or significant changes in lithology provides the initial coarse filter for further assessment of lower levels of organisation, channel type for example. The division is necessary, as research has demonstrated that not all macro-reaches respond in the same way to disturbance or stress, nor do they have the same biotic assemblages. Four statistical methods (Von Neumann mean square error, CUSUM plots or unweighted values and the Worsley Likelihood Ratio Test (WLRT)) were used to define macro-reach breaks for four South African rivers (Crocodile, Olifants, Mhlathuze and Seekoei Rivers) and were compared to previously defined macro-reach delineations based on expert-driven approaches. Results indicate that the CUSUM and WLRT approaches most closely match the macro-reach breakpoints as defined by the expert-driven approach. An automated desktop procedure was developed for computing statistically defensible, multiple change points along profiles using an adaptation of the WLRT method. The adapted approach does not require an a priori knowledge of the break points, as is the case in other applications of the WLRT. It is concluded that the adapted WLRT approach can be used with a reasonable degree of certainty where there are insufficient data and/or where a regional or national assessment is required that is repeatable and statistically defensible. Where possible, however, there is no substitute for primary data collection, field work and a detailed expert-driven approach.