River channels are dynamic systems that continuously adjust their morphology in response to varying inputs of sediment and water over a range of timescales. These adjustments profoundly influence ecological processes, hydrological dynamics, and geomorphological stability. Understanding the lateral response of alluvial rivers to sediment pulses—transient increases in sediment supply due to natural events like landslides or anthropogenic activities such as mining—is crucial for assessing river channel resilience and managing associated hazards.
This thesis investigates the morphological changes in river channel planform induced by sediment pulses, focusing on active channel width, braiding intensity, and area of bars as proxies for understanding how rivers accommodate and adjust to increased sediment loads. High-to-medium resolution imagery and historical aerial photographs spanning multiple decades are used to track temporal changes in channel morphology. Case studies on the Lillooet River, Canada, and the Puquiri-Colorado and Inambari rivers, Peru, allow an analysis of how different natural and anthropogenic sediment sources and landscape characteristics influence river channel responses. These findings contribute to a deeper understanding of the geomorphic processes governing alluvial rivers under varying sediment supply regimes, highlighting important watershed management challenges related to ongoing and future environmental change.
The short-term response of Lillooet River, where significant changes in planform morphology were observed following a 2010 landslide, reveals the river reverting to its pre-disturbance braiding intensity within 3 to 5 years after the event. However, increased active channel width and bar area persisted for longer, with higher magnitudes observed in the final year of analysis (2022) compared to the period before the landslide. Factors such as time since the landslide, water yields, and valley morphology influenced the lateral response of Lillooet River. The longer-term response of Lillooet River, from 1947-2010, also shows changes in active channel width influenced by the cumulative volume of sediment supplied from landslides, hydrological regime, and glacier dynamics. Consideration of the longer-term evolution of lateral channel response is thus essential to place the impact of discrete events, such as landslides, in context.
Finally, analysis of the Puquiri-Colorado and Inambari rivers shows ongoing alluvial gold mining to have altered the original valley bottom, introducing vast quantities of sediment into the river and facilitating storage of this material within its floodplain (Figure 1). The study reveals the substantial geomorphic impact of artisanal gold mining operations, with the Puquiri-Colorado River influenced the most.
The methodologies and results illustrated in this research provide a blueprint for future studies to evaluate and manage riverine ecosystems in the face of escalating human activities and climate variability.
