The analysis of time variability of riverbed elevation recorded at four cross-sections of two rivers: Raba and Dunajec was made. The causes of very intense erosion were pointed out, namely: intensive sediment extraction, inadequately carried out river training works including shortcuts and narrowings of riverbeds, and constructions of hydro-technical facilities, disrupting the continuity of sediment transport along the river length. Forecasting was carried out using a local weighted regression method, LOESS. In order to examine how far the length of the historical series influences the quality of forecasts, a comparison was made between the 5-year forecasts based on the 5-year history and the 5-year forecasts based on the 10-year history. The results suggest the need for preliminary data analysis before using a particular version of the model in or-der to take into account the type of variation exhibited by the historical series.