The decrease of river runoff draining into Lake Balkhash in Central Asia was investigated using hydrological and meteorological data over a long-term period. The data from the difference integral curves of the annual runoff from 1911 to 1986 suggested that a low-flow period began in 1970 in the River Ili, and in 1973 in the east rivers, continuing until 1986. Compared with the runoff before 1969, the decrease of runoff in the upper reaches of the River Ili was less than those in the middle and lower reaches. The decrease in the upper reaches resulted from natural variability, whereas those of the middle and lower reaches were due to a combination of the effects of climate and human activity. There are two reasons for this. First, after 1970, precipitation did not decrease uniformly across the entire basin. Second, in 1970, the Kapchagay dam started operating. As a result, the annual inflow into Lake Balkhash from the River Ili after 1970 decreased to 77% of the pre-1969 mean. Similarly, after 1973, the inflow from the east rivers was 75% of the pre-1972 mean.
The characteristics of the runoff in the growing season (April to September) and those in the non-growing season (October to March) were investigated for the upper, middle, and lower reaches of the River Ili. In all reaches, the runoff in the non-growing season remained unchanged both before and after 1969. However, from 1970 to 1986, decreases of river flow in the growing season were 2·0 to 2·5 times greater in the middle and lower reaches than in the upper reaches. This was primarily due to the impacts of human activity. In conclusion, it can be deduced that the decrease in runoff after 1970 in the middle and lower reaches was the result of human activity during the low-flow period.