Assessing impacts of climate change and river regulation on flow regimes in cold climate: A study of a pristine and a regulated river in the sub-arctic setting of Northern Europe

River regulation and climate change are two major causes of river regime alteration that are difficult to separate, and thus the individual effect of each is not always clear. To demonstrate the relative effect of regulation practices and observed climate change, we present results from a spatio-temporal study of two adjacent rivers in Northern Europe, the Kemijoki (regulated) and the Tornionjoki (pristine), with similar climate and catchment conditions. Spatial and temporal analysis was performed using daily hydrologic data from 11 gauging stations on these rivers. Flow regime alteration analysis showed that both rivers had altered significantly on a daily scale, but that there were low impacts on flow regime on a monthly scale. Daily variation in natural stream flow regime for time period of 2001–2015 was altered in both rivers when compared to the discharge during 1952–1966, but the degree of hydrologic alteration (DHA) of daily flow values for the winter months was significantly higher in the case of the regulated Kemijoki river. For river Tornionjoki, changes and variations related to climate seems to be the reason for the observed flow regime change with increased mean monthly flow for August and December. An increase in annual maximum and decrease in minimum flows was also observed in Tornionjoki, these changes however were smaller than overall changes in the regulated river Kemijoki. Based on the degree of alteration (Indicators of Hydrologic Alteration (IHA) analysis), both climate change and regulation operations affected the river regimes. About 50% of total change in observed range variability approach (RVA) in the river Kemijoki was estimated to result from regulation practices. This was confirmed by analysing data from Ounasjoki which is the pristine tributary of Kemijoki. The extent of alteration in both rivers was dependent on the spatial position of the gauging station within the river network. The impact of regulation on downstream reaches of the regulated river decreased with the increase in catchment area around that part of the river. Thus integrating daily and monthly flow data from varied spatial location on a river network, offers a useful tool for analysing flow regime conditions.