Hauptarm

Hauptarm is a term used in hydrology and fluvial geomorphology to denote the principal channel of a river or delta distributary system. It is distinguished from Nebenarmen, the side arms or secondary channels, and is typically the channel that carries the largest discharge and forms the main conduit for water and sediment downstream.

Identification and dynamics of the Hauptarm can be fluid over time. Which channel is considered the Hauptarm may change due to natural processes such as avulsion, where a sudden shift in flow makes a different channel the dominant one, as well as long-term sedimentation, erosion, and human interventions like river engineering. In maps and models, the Hauptarm is often defined by metrics such as highest mean annual discharge, greatest cross-sectional area, or the channel designated as the primary watercourse in a given reach.

Formation and behavior in delta and river systems involve a branching network of channels that distribute flow. The Hauptarm typically remains the deepest and widest channel, but its position and size can migrate. Ecologically, it often supports higher flows and different habitats compared to Nebenarme, influencing navigation, flood management, and sediment transport.

In German-language literature, Hauptarm is commonly used to describe the main flow path within a river or delta system. The term is frequently paired with Nebenarm to characterize the entire distributary network. See also Nebenarm, distributary, avulsion, and deltaic processes.