Disturbances in coarse bedload transport in a formerly glaciated catchment

Dynamics of bedload transport increases with an increase of hydrologic parameters, i.e. stream power, water velocity and discharge. In mountain stream channels, these parameters increase in the downstream direction, and therefore the dynamics of bedload transport increase down the channel. This pattern may become altered temporarily or over long periods of time due to local extreme events or human impact. Here, we identify disturbances in bedload transport in the formerly glaciated catchment located in the Western Tatras in Poland. We then determine the role of disturbances in system connectivity and sediment transfer. Bedload transport measurements in the Chochołowski catchment were performed in the period 1975 to 2018. The said process occurred as many as triggered up to several times a year. However bedload became activated along the entire length of the channel system (10.5 km) every 2 to 5 years when the stream discharge exceeded 10 m³ s^-1. The distance of bedload movement during such events was between 12 m in headwaters and over 100 m in the lower reach. In such situations, bedload dynamics increased downstream, and stream power was sufficient to overcome local barriers (i.e. boulder and log steps). Downstream increases in bedload dynamics was disturbed and inverted by sudden snowmelt and locally heavy rainfall. These types of events caused the dynamics of bedload transport to be up to 320% greater in the upper part of the studied catchment and to decline in the downstream direction over a distance of 7 km to yield an attenuation effect. The same pattern was observed in local tributaries where the dynamics of bedload transport were 200% higher in the upper part and attenuation was observed along a distance of 1.5 km in the downstream direction. All events of this type cause seasonal disturbances in bedload transport. However, the greatest effect on bedload dynamics was produced by natural deforestation. A 16% decrease in forest cover causes a fluvial system disequilibrium manifested in an intensification of hydro-geomorphologic processes and formation of new landforms.