The Duriangkang reservoir was built in 1992–1995. It was the first estuary dam built in Indonesia and has the largest storage of any dam in Batam City. Its volume of effective storage is 107 million m3, and it is fulfilling the water need at a rate of about 2,500 l/s. The Muka Kuning reservoir was completed in 1990 and began operating in 1991. Initially, the IPA Muka Kuning reservoir had a water production rate of 310 l/s, which increased to 600 l/s in 2015 because of Batam City’s increasing water needs. This research is aimed at assessing the water availability in the Duriangkang–Muka Kuning cascade reservoir system. This evaluation was carried out to support the integrated management of optimization in cascade reservoirs. In this cascade system of the Muka Kuning and Duriangkang reservoirs, water is taken from downstream into the upstream reservoir, and a pump is used to create inflow into the downstream reservoir. This study’s methodology comprises rainfall data tests, trend absence tests, stationery tests, variance stability tests, and mean stability tests. Moreover, we conducted an analysis of dependable discharge for the wet discharge condition (35% probability), the normal discharge condition (50% probability), and the dry discharge condition (65% probability). To evaluate the water availability, we used the reservoir water balance. The results show that the water availability in Duriangkang is enough to fulfill the water need under all conditions (wet, dry, and normal); however, for Muka Kuning, the water availability can be fulfilled in the wet and normal conditions only, and under the dry condition, the water availability cannot fulfill the water need. Therefore, ensuring the integrated management of optimization is necessary.