The efficiency of the river machine

Abstract

Defining a characteristic bedload transport efficiency using all the data available for a river, or data specifically relating to a discrete timeframe with distinctive hydrosedimentological characteristics, is a necessary prerequisite to any attempt to investigate the response of the river machine to changes in sediment supply and availability. This characteristic efficiency encapsulates variability between rivers that arises because the aggregate transport rate and size of sediment in motion reflect the first order control exerted by sediment supply and the available power, as well as diverse factors, including channel properties such as armoring, that have an immediate influence on sediment availability. There are well-defined upper and lower size-dependent limits to the efficiency of the river machine and three distinctive efficiency metapopulations are readily apparent; which are indicative of rivers with live, armored and heterogeneous beds. Time-variations in efficiency are most pronounced when the sediment supply is limited, and transport rates are regulated by the response of the bed surface to the flow and/or the amount of entrainable material held in storage. To properly differentiate between rivers the size of the material in motion and the water depth must be specified. Knowledge of how stream power responds to changes in water surface slope and flow velocity, as well as information about concurrent topographic and textual adjustments to the bed, is also required to elucidate the time-variations in efficiency that occur in rivers with a steep and highly variable longitudinal profile.