In Latvia, rivers are characterised by a typical hydrograph ofEastern Europe: two main discharge peaks - during the spring snowmelt and in late autumn during the intensive rainfall, and low river discharge - in winter and summer. This study deals with long-term changes in low and high discharges of the five bigger Latvian rivers Daugava, Venta, Lielupe, Gauja and Salaca. The mathematical statistical methods were applied in the analyses of daily river discharge data series from 1881 to 2009. The low flow periods were defined as a series of the 30-day minimum discharge in a cold period (Q30_cold, December - February) as well as warm period (Q_30warm, June - October). The high flow period was characterized by maximal discharge of the year (Q_max) which was mostly observed in the spring flood period (March - April) and sometimes occurred also in other seasons of the year. The coefficient of uneven runoff distribution d was calculated which expresses the distribution of river runoff per year. The large-scale atmospheric circulation like the North Atlantic pattern and it influence to the streamflow was studies as well.

The study results reveal that statistically significant trend of increase for low flow of cold period and significant trend of decrease for the high discharge and coefficient d of uneven runoff distribution were found. Statistically significant upward trend in the 30-day minimum discharge of warm period were identified for the Venta and the Lielupe representing Western and Central hydrological districts respectively. Changes in river hydrological regime are mainly caused by climatic factors. Therefore, Latvian river hydrograph has changed in the turn of the 20^th and 21^stcentury and become more similar to Western European rivers.