RIVERS AS ARCHIVES OF PALEO-PRECIPITATION PATTERNS

Fluvial systems commonly experience hysteresis and complex signal buffering effects that complicate tracking of allogenic vs autogenic forcing factors. This paper presents a study of 52 modern and ancient fluvial datasets with river dynamics dominated by highly seasonal precipitation patterns, such as in monsoonal domain and the bordering arid to sub-humid climate zones.

Rivers that receive significant amounts of their surface water supply from monsoon precipitation characteristically experience seasonal floods, and display seasonally highly variable discharge, controlled by the monsoon trough passage and its related cyclones. The intense summer rainfall causes high-magnitude floods, whereas rivers only transmit a low base flow during the dry winters. Also for many rivers in the sub-humid to arid subtropics, bordering the monsoon domain, the monsoon rain is the main source of surface water recharge. However, such rivers may receive monsoon rain and transmit discharge only during abnormal or strengthened monsoon seasons. This annual discharge variability or range, as compared to the mean annual discharge, distinguishes the monsoonal and subtropical rivers from the rivers in equatorial tropics and temperate perennial precipitation zones, where the annual range is relatively small compared to the annual mean discharge.

This paper explores the effects of this seasonal and yearly variable rainfall, and the resultant highly peaked discharge pattern on river morphodynamics, and presents a comparison of modern and ancient monsoonal and subtropical river deposits. The review demonstrates that seasonal and inter-annual precipitation range is a key control on river morphodynamics and the resultant sediments.

Sediment transport and deposition in such rivers occurs during limited seasonal events that exhibit high water discharge and Froude supercritical flow velocities, resulting in dominant suspension transport mode even for sandy and gravely sediments. These latter morphodynamic processes minimize hysteresis and buffering effects and rivers with highly seasonal discharge thus provide reliable proxies for interpreting paleo-precipitation regimes.