present. The presence of the large delta is explained by the formation and rejuvenation of the hilly region near Samarinda. Climate The Mahakam catchment is around the equator. The average annual rainfall in the catchment area is 3163 mm. The average runoff is around 1911 mm. According to Köppen climate classification, this area belongs to type Af (tropical rainforest) that has a minimum temperature ≥18 °C and precipitation of the driest month in normal year ≥60 mm Transfer of mass and energy in the tropical zone occurred through general air circulation known as the Hadley cell. According to Seidel et al. (2008) the precipitation pattern in this area is largely determined by this large-scale atmospheric wind patterns, which is observable in several ways throughout the atmosphere. This circulation carries moisture into the air, generating rainfall in equatorial regions, whereas at the edges of the tropical belt are drier. Within this circulation, evaporation occurs intensively around the equator on the center of low pressure called the Intertropical Convergence Zone (ITCZ), characterized by accumulation of cloud in the area. The ITCZ moves following the pseudo-motion of the sun within 23.5°N and 23.5°S zone, therefore its position always changes according to this motion. The ITCZ drives the Indo-Australian monsoon phenomena which influence the regional climate including the Mahakam catchment. In December, January, February (winter in Northern Hemisphere) the concentration of high pressure in Asia and low pressure in Australia make the west wind blows in Indonesia (west monsoon). In June, July, August concentration of low pressure in Asia (summer in Northern Hemisphere) and concentration of high pressure in Australia make the east wind blows in Indonesia (east monsoon). Due to the global air circulation and the regional climate mentioned above, the Mahakam catchment which is located around the equator has a bimodal rainfall pattern with two peaks of rainfall, which are generally occurred in December and May. This is because the ITCZ passed through the equator twice a year, from the Northern Hemisphere in September and from the Southern Hemisphere in March. Lakes There are about 76 lakes spread in the Mahakam river basin and about 30 lakes are located in the middle Mahakam area including the three main lakes (Lake Jempang 15,000 Ha; Lake Semayang 13,000 Ha; Lake Melintang 11,000 Ha). The lakes levels are seasonally fluctuated from 0.5 m – 1 m during dry period to seven meters during rainy season.