Longitudinal variations in channel morphology of an ephemeral stream from upland to lowland, Daihai Lake basin, North China

Abstract Recognizing variations in channel morphology of ephemeral streams and their controls is important for environment management and protection, and even for paleoenvironment reconstruction. Insights gained from modern ephemeral streams in various hydroclimatic or tectonic settings are a key to elucidating the relationship between channel morphology and genetic processes. In this study, we present a small ephemeral stream in a semi-arid area with influence of the East Asia monsoon in North China. Based on field work and high-resolution topographic data collected by unmanned aerial vehicle (UAV), this study reveals detailed geomorphological characteristics of the ephemeral stream from upland to lowland, which is divided into five segments. The channel dimension exhibits an overall increasing longitudinal trend in the upland and a general decreasing trend in the lowland, but locally, the lowland channel morphology fluctuates quasi-periodically. Quantitative analysis of channel morphological parameters and related factors is conducted to discuss the controls on channel morphology. Variations in the upland channel morphology are directly influenced by discharge/drainage area, composition of substrate, and sediment flux, and indirectly affected by slope. Ephemeral floods in the uplands, associated with an unconsolidated substrate that produces a high sediment flux, are capable of generating larger channels than the case for perennial flows in the bedrock-dominated upland. In the lowland, fluctuation in channel reach morphology is an integrated result of the local flow regimes, vegetations, and anthropogenic activities. Froude supercritical flows associated with hydraulic jumps are inferred as the primary cause of the alternating channel geometry in the lowland. Through this study, we highlight the significant role of intensive floods derived from monsoon precipitation and abundant sediment supply in forming large-scale ephemeral stream channels, yet details of flood process are needed for further exploration.