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Danau Sentani
A lake created by a fault in the earths' crust

Lake Sentani, near Jayapura at the northeastern extremity of Papua, lies at an elevation of 73 m in a fault-controlled depression mainly in Mesozoic mafic and ultramafic rocks of the Cyclops Ophiolite Belt. It is bounded by the Cyclops Mountains block to the north and the lower-standing terrain of the New Guinea fold thrust belt to the south. An irregularly shaped body with approximate maximum dimensions of 28 km (E-W) by 19 km (N-S) and a surface area of 10,400 ha, Lake Sentani is by far the largest of the Papua lakes. It is fed by a catchment area of about 600 km2 and has one outlet only, via the Jafuri and Tami rivers to the Pacific Ocean near the Papua New Guinea border.

The lake is divisible into three main sectors with maximum recorded depths of 7 to 52 meters. Average annual rainfall around the lake is about 2 meters and lake level fluctuates about 0.4 m with seasonal variation in inflow. The lake is widely believed to have evolved by the tectonic damming and uplift of an arm of the sea, but such a connection has not been demonstrated.

Because of its proximity to the provincial capital and the large population around it, Sentani is the best studied of Papua lakes. According to surveys in 1970-71, 1984 and 1987 the lake is thermally unstratified, with temperatures of 29-32 C in the top 10 m. Surface pH is 6.2-6.8 and, on the basis of turbidity, plankton levels are low at 1-2 mg/L except in the westernmost basin, where circulation is limited, turbidity is doubled and seasonal algal blooms, with resultant fish mortality, have been reported.

The most recent survey (Renyaan, 1993) recorded 33 species of fish, of which 12 are indigenous, 8 anadromous and 13 introduced. Surveys over a 1 year period have shown an increase in introduced species but the impact on the total fish population has not been documented. Sawfish (Pristis microdon) up to 3 m or more were well known in the lake until the Seventies and are a common motif in traditional Sentani art, but appear to have become extinct. Fish are extensively raised in ponds and cages around the perimeter of the lake and the introduction of species (particularly carp and tilapia) has been both accidental and intentional.

Preliminary bottom sediment samples from the eastern part of the lake have recently yielded sparse populations of arcellacean microfauna, dominated by Centropyxid types. In North America these species are found associated with brackish or polluted water conditions, raising the possibility that there is residual salinity in the deeper parts of the lake. Many of the Sentani people, who inhabit the islands, perimeter and environs of the lake, still have a traditional subsistence economy based on fishing and sago harvesting. This has been sustainable for centuries but local reports suggest that catch yields have diminished in recent years. Whether this is a result of overfishing (as a result of population growth and/or market pressure), pollution or introduction of foreign species is not established.

Many of the residents occupy dwellings built on posts over the lake, which thus serves as a depository for sewage, leading to locally high coliform counts but also to nutrient enrichment. Water hyacinth (Eichornia crassipes), introduced since the early Seventies, has become a major plant pest and may be contributing to decline of some species. Much of the mountainous terrain between the north shore of the lake and the ocean falls within the Cyclops Strict Nature Reserve. The future management of the reserve and buffer zone, and the environmental quality of the lake, are strongly interdependent. Recently a major reforestation project of grassland on the slopes surrounding the lake has been initiated, with the support of forest companies operating in Papua.

A major sustainable development issue for the inhabitants of the lake and surroundings is the existing proposal to build a hydroelectric generating facility, by means of a dam on the Jafuri River to divert the lake drainage eastward through a canal to a power station and thence to an outlet at the sea in Yotefa Bay near Jayapura. Several feasibility and environmental impact studies have yet to totally define the cost/benefit consequences of this project.

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