River temperature and the thermal-dynamic transport of sediment

Abstract

Spatial and temporal variability of river temperature, and its impact on sediment transport, is explored. Global river temperatures, computed daily using 1980–2010 climate, reflect latitudinal and seasonal variations in solar radiation, wind patterns, humidity, and water sources, and can be cooler or warmer than air temperatures. Subpolar, Polar and High Plateau rivers are cold (TMEAN: <8 °C), and of intermediate variability (Tσ: 6° to 10 °C), are highly sensitive to temperature variations, which together favor a river's ability to transport sediment. Temperate, Subtropical and Alpine rivers are of intermediate temperatures (TMEAN: 5° to 26 °C) with a maximum in variability (Tσ: >8°); their transport responsiveness is dependent on their minimum temperature. Tropical rivers are warm (TMEAN: >25 °C), less sensitive to temperature swings, and possess a reduced sediment transport capacity; their narrow temperature variability (Tσ: 1° to 4 °C) favors finer-grained sediment transport. Fluid density of river water ranges by 3.5%, and more rarely increases by 170% and thereby increases sediment transport 2.8-fold. Kinematic viscosity has a global range of 72.5%, and works to reduce sediment transport as the river warms. A 25 °C increase in water temperature, due to some combination of river elevation descent, or flow to warmer latitudes, or seasonal increases, can increase grain settling velocities by 7.3%, 42%, 85%, 90% for a 1000-μm, 250-μm, 62.5-μm, or a 10-μm grain respectively, and reduce sediment transport by 90% for a grain size population centered at 62.5 μm, or 300% if centered at 10 μm. The common practice of using fluid density and temperature constants (ρf = 1000 kg/m3, T = 20 °C) in transport models, can greatly under-predict subpolar and alpine river transport and over-predict sediment transport in a tropical river. Temperature variations impact rivers that transport a preponderance of fine sediment, a characteristic of most large rivers. These trends in river temperature and sediment transport have important ecological and anthropogenic implications. The effects on sediment transport will be especially important to quantify under a warming climate and where human influences, such as urbanization, may change the riverine thermal regime.