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<Technology Needs for Lake Management in Indonesia - Investigation of Rawa Danau and Rawa Pening, Java> C. Assessment of Hydrological Conditions in the Caldera No explanation is offered in the available literature for these features of the river drainage system, nor indeed is any mention made of them in the available literature. It appears that they have been accepted as the normal situation in this caldera. This is despite the fact that at least 16 independent surveys have been carried out in the caldera and "Cagar Alam" (Nature Reserve) since the work of Endert (1932). The fact that most of these surveys have been undertaken by biologists does not excuse the failure of the ecologists to understand the implications of these features of the caldera. There appears to have been no appreciation, since that of Endert in 1932 - with the possible exception of J. Wind (1977) - of the possibility that the exit from the caldera has been altered to allow more water to drain out, nor is there any mention, even by Endert, of any possibility that channels were dug to accelerate drainage of water from the swamp. Detailed documentary proof of these alterations to the natural environment of 200 years ago is lacking. Details are also lacking of the extent and depth of the original lake. The relevant information may be available in Government records in The Netherlands. The structure of the original outflow is unknown, and cannot be readily determined as the bedrock at the present exit from the caldera is covered with deep deposits of loose rock presumably excavated in lowering of the bed of the river. It seems likely that while lowering the river bed the river water would have had to be diverted. So it is likely that the original path of the Ci Danau from the caldera has been considerably altered. It is likely, by comparison with the structure of similar caldera outflows, that the original outflow from the caldera was a shallow sheet of water stretching from one side to the other across small valley at the exit from the caldera. This is about 200 to 250 metres wide. The original height of the outflow is assumed to be the height of the bed rock with evidence of extensive water erosion, two metres above the level of the present outflow . It is possible that, under natural conditions, over 200 years ago, the Ci Danau river did not flow from the caldera at times during the dry season. In that case, the water level would have fallen below the original lip of rock, two metres above the present outflow. At times during periods of heavy rain, the water level in the Rawa may have risen appreciably above the two-metre height of the original lip. The exact amount of such increased water height might be calculable from data on rainfall input per hour in the catchment, versus the cross sectional area of the original outflow. If these assumptions are correct an estimation of the possible depth of water flowing from the caldera, could be made based on visual inspection of the site and examination of the existing data. It is unlikely that a height of 3 metres above the present water level was reached for any length of time more than two days. It seems unlikely that the water from the caldera ever reached a height of more than one metre in depth over the width of the original outflow. Following the lowering of the base of the outlet from the caldera, the water in the Ci Danau river has rapidly eroded through the sediments of the caldera to form a river channel with a strong flow of water, rather than a smooth, flat, and relatively slow flow over the surface of the sediments in the caldera. The depth of the river channel would be exactly the same as the depth of the exit from the caldera. The river channel, with steep unstable banks, would erode backwards into the sediments of the caldera, because of the increased speed and erosivity of the water as it flowed down the gradient to the new, lower, exit level. This process has been going on for many years and has now reached upstream beyond the junction of the Ci Danau and the Ci Bojong rivers. There has been severe backward erosion into the bed of the Ci Bojong, leaving the banks of this stream unstable in places. Houses from several kampungs cluster along the top of the steep banks of this very small stream, in what is unlikely to be a safe situation in the long term. It is likely that these kampungs were originally constructed on the material excavated from the drainage channels. The land surface near the drainage channels would naturally be well drained, dry and stable, and this would be increased by the additional material from the excavation of the channels. However as the drainage channels erode downwards through the stiff clay sediments of the caldera, the houses are increasingly perched on the edge of a steep drop down to a small stream, during the dry season, and on the edge of a deep raging torrent during the wet season. The banks of the stream are increasingly unstable and have collapsed in places. Increased gradients in channels in the caldera which ultimately lead to the lower exit level of the Ci Danau from the caldera will lead to higher rates of flow in these channels. This may be the cause of the surprisingly high rates of flow encountered at the junction of the Ci Kalumpang and the Ci Danau rivers. Therefore, it maybe concluded that - 1. Two hundred years ago, a much larger area of land was permanently covered by water in the caldera than is covered at present. This area would have formed the lake. 2. The water in the lake would have covered the sediments over much of the caldera and probably reached a height, during the dry season, of at least two metres above the level of the present outflow. 3. The water varied seasonally in height, and the lake probably acted as a slow release valve on water from the catchment. 4. The water in the lake would have largely prevented the soil in the area of the swamp forest from drying out.
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