Artificial riffles as a driver of bed morphology enrichment in a trained upland river

Artificial riffles are increasingly used in river renaturalization to stabilize incised channels and enhance habitat diversity. However, their long-term structural stability remains under-researched, particularly in reservoir-impacted systems. This study evaluates the nine-year persistence of 12 artificial and 4 seminatural riffles in the regulated upland Bóbr River (Poland) using RTK GPS field surveys, hydraulic and grain size measurements, and hydromorphological assessments (River Habitat Survey). The results demonstrate a clear divergence in evolution based on construction quality. Well-executed riffles (n = 6) maintained high morphological and hydraulic stability (p > 0.05) throughout the monitoring period (2015-2023). In contrast, poorly-executed structures (n = 10) underwent significant degradation, characterized by a significant increase in mean channel depth (p = 0.037) and flow velocity (p = 0.014). In these structures, the winnowing of finer fractions led to a coarsening of the substrate, with the median grain size of the riffle runs increasing from 35 mm to 78 mm, resulting in a fragmented 'dragon's teeth' morphology. While artificial riffles proved to be a durable and nature-friendly alternative to traditional bed-stabilization works, their long-term success is strictly dependent on adherence to design grain size distributions (D₅₀, D₈₄) and proper substrate mixing. These findings provide critical technical benchmarks for the sustainable renaturalization of incised, gravel-bed upland rivers.