Rivers are usually perturbed by natural disturbances. Human activities, either in the catchments of streams or in their channels or in both, can generate disturbances that change streams and their biota. To understand the impacts to streams of disturbance, regardless of origin, it is necessary to monitor. It is also crucial that there is a clear understanding both of the type of disturbance and of the purpose(s) of monitoring.

TYPES OF DISTURBANCE

The major reason for monitoring flowing-water ecosystems is to detect the effects of perturbation. A perturbation to a population, community or ecosystem occurs when there is a distinct and abnormal change to properties of the system due to disturbance (Bender et al. 1984). A perturbation consists of two events: the disturbance, which is the application of the disturbing force (e.g. flow reduction) or agent (e.g. pollution) to the biota of the system; and the response of the affected biota to the disturbance (Glasby & Underwood 1996). It is important to separate the application of a disturbance from the consequential biotic responses (Glasby & Underwood 1996; Lake 1990, 2000). This allows comparisons to be made of the differential responses of individuals, populations and communities of organisms to similar disturbances and to different types of disturbances. It is also important to note that there may be a considerable time lag between the disturbance and the consequential response by the biota.