This study systematically analyzes long-term (1936--1999) monthly discharge records for the major subbasins within the Lena River watershed in order to document significant streamflow hydrology changes induced by human activities (particularly reservoirs) and by natural variations/changes. The results show that the upper streams of the watershed, without much human impact, experience a runoff increase in winter, spring, and (particularly) summer seasons and a discharge decrease in fall season. These changes in seasonal streamflow characteristics indicate a hydrologic regime shift toward early snowmelt and higher summer streamflow perhaps due to regional climate warming and permafrost degradation in the southern parts of Siberia. The results also demonstrate that reservoir regulations have significantly altered the monthly discharge regime in the lower parts of Lena River basin. Because of a large dam in west Lena River, summer (high) flows at the outlet of the Vilui valley have been reduced by up to 55% and winter (low) flows have been increased by up to 30 times. These alterations, plus streamflow changes in the upper Lena regions, lead to strong upward trends (up to 90%) in monthly discharge at the basin outlet during the low-flow months and weak increases (5--10%) in the high-flow season. Monthly flow records at the basin outlet have been reconstructed by a regression method to reduce reservoir impacts. Trend analyses and comparisons between the observed and reconstructed monthly flows show that because of reservoir regulations, discharge records observed at the Lena basin outlet do not always represent natural changes and variations. They tend to underestimate the natural runoff trends in summer and overestimate the trends in both winter and fall seasons. Therefore the cold season discharge increase identified at the mouth of the Lena basin is not all naturally caused, but the combined effect of reservoir regulation and natural runoff changes in the unregulated upper subbasins. This study clearly illustrates the importance of human activities in regional and global environment changes and points to a need to examine human impacts in other large high-latitude watersheds.