Hydrological characteristics of Mahakam cascade lake as a reference for the sustainability management

Mahakam cascade lakes consist of Lake Semayang, Melintang, and Jempang is a floodplain lake. Mahakam cascade lakes include the 15 national priority lakes which are the most urgent situation. This lake is characterized by dynamic water level fluctuation, resulting in high fishery productivity. The lake’s hydrological characteristic is the main component in controlling the lake ecosystem, therefore it is crucial to evaluate the influenced factors. The objective of this research is to describe the lake’s hydrological characteristics, including inlet-outlet discharge, sedimentation, water level pattern, and land cover change in the catchment area. Further, provide a reference for the conservation of lake ecosystems. We performed a field campaign to measure river discharge and collect water samples for sediment-load laboratory analysis. We analyzed daily water level data recorded from 1989 to 2010. We obtained the time series land cover from Landsat data and performed GIS spatial analysis. Our results revealed that first, the Rebak Rinding River has the biggest contribution to lake sedimentation (3,859 tons/day, 77% of the total sediments entering the lake). Second, the water level tended to increase during the medium and extreme flood seasons. Contrary, a relatively small decrease in the water level during the medium-receding season. Third, the catchment area is 51,573 km2 (66 times larger than the lake water bodies). From 2000 to 2019, the forest decreased by 2,063 km2. As a consequence, increased erosion-sensitive area by 2,695 km2. We emphasized preserving the catchment area is vital to reduce the sedimentation rate and water fluctuation. We recommend the Rebak Rinding sub-catchment as the priority area for controlling deforestation, and erosion on the plantation, mining, and agricultural area.


Introduction
Indonesia is a country that has more than 500 large lakes with various types of lakes [1].The process of lake formation is caused by volcanic, tectonic, and river activities, as well as a combination of these activities, so lakes have different hydrological characteristics.The hydrological characteristics include hydrometry, morphometry, fluctuations in water level, and the ratio of the lake surface area to the catchment area (CA).Flood lakes are generally located in low-lying areas that have dynamic water level fluctuations so that biological productivity is high.While volcanic lakes are generally in the highlands, the water depth is large, the water residence time is long, and the temperature fluctuations 1260 (2023) 012032 IOP Publishing doi:10.1088/1755-1315/1260/1/012032 2 are relatively small so that water stratification occurs (thermocline) [2].It is the specific characteristics of these lakes that need attention in lake management so that the results are more optimum.
The Mahakam cascade lake which consists of Lake Jempang, Lake Semayang, and Lake Melintang has been designated by the Government of Indonesia as one of 15 national priority lakes whose condition is critical so that it receives priority to be handled (Agreement on 9 Ministries at the 1st Indonesian Lakes Conference in Bali, August 2009) and is currently being strengthened by Presidential Decree No. 60 of 2021 concerning Saving National Priority Lakes.This cascade lake functions as a flood controller, microclimate controller, habitat for dolphins, fishery resources, and no less important as controlling the surrounding groundwater system [3].However, the current condition of the lake is experiencing environmental degradation which is characterized by high sedimentation, changes in extreme water level fluctuation patterns, decreased water quality, decreased fishery productivity, and disturbed biodiversity [4].
This cascade lake is a flooded lake with very high fishery productivity.This high fishery productivity is highly dependent on the fluctuation pattern of the lake area.Extreme floods and droughts are becoming more and more frequent.When a big flood occurs for a long time and vice versa when a drought occurs it also lasts a long time [5].This extreme pattern of lake theme fluctuations hurts the function of the lake waters as a habitat, where floods sweep away habitat resources, and when the water recedes with a long dry period, this condition is no longer by the order of the aquatic life cycle that must be lived.
Changes in lake water level fluctuations also change the structure of the vegetation that grows in the lake waters, thus threatening the sustainability of the lake ecosystem.So fluctuations in lake water level are the main process that affects the structure and function of ecosystems in lakes [6].According to [7], increased fluctuations in lake water level will affect the littoral zone and the subsequent effect on habitat diversity will decrease, consequently affecting the number and abundance of species.
The sedimentation rate of this cascade lake is quite high, around 10 -20 mm/year [8].The effect of this sedimentation in the long term reduces the water holding capacity, thereby reducing the performance of the lake in controlling flooding and disturbing the lake ecosystem.Soil erosion from upstream areas that enter lake waters is not only in the form of sediment but also in the form of nutrients and metal elements that can increase the eutrophication of lake water and ecosystem degradation [9], which in turn accelerates the growth of aquatic weeds.
The sedimentation problem is closely related to the damage to the hydrological system in the lake catchment area due to land conversion and agricultural cultivation that does not heed the aspects of soil and water conservation.Changes in land use from forests to plantations, mining, and agriculture have led to changes in the hydrological cycle and high sedimentation rates of lakes, thus damaging the ecosystem [10,11].
This Cascade Lake hydrological system includes subsystems in lake waters and subsystems in catchment areas.The pattern of discharge and sediment entering the lake is strongly influenced by the environmental conditions of the catchment area.Flow characteristics and sediment material content of river water are influenced by rainfall and hydrological processes in the catchment area.The hydrological process is determined by land use, soil properties, topography, soil surface conditions, geomorphology, and anthropogenic activities.Meanwhile, in lake waters a separate hydrological process occurs, besides being influenced by the water yield of CA, it is also influenced by water quality and meteorological elements.Therefore, analysis of hydrological aspects is very necessary for developing the concept of lake ecosystem management.This study aims to analyze the pattern of lake water level fluctuations, water, and sediment discharge entering and leaving the lake, and land cover changes in the lake catchment area.

Research sites
The location of the Mahakam cascade lake which consists of Lake Jempang, Lake Semayang, and Lake Melintang laid in the areas of Kutai Kartanegara Regency and West Kutai Regency, East Kalimantan Province (Figure 1).Meanwhile, the lake catchment area reaches an even wider area, adding 2 regencies but still in the same province, namely: Malinau Regency and Mahakam Hulu Regency.

Data Collection and Analysis
The data required in this study consists of primary data and secondary data.Secondary data were obtained from government agencies, namely: the Ministry of Public Works and Public Housing, Ministry of Environment and Forestry, Meteorology and Geophysics Agency, and Indonesian Institute of Sciences, both in raw data and scientific reports.While the primary data obtained from observations/measurements in the field and analysis of water samples include: river water discharge.Measurement of river water discharge was carried out on several rivers that enter and leave the lake.The rivers that enter Semayang-Melintang Lake are the Rebak Rinding, Enggelam, and Kahala Rivers; and the one that comes out of the lake is the Pela river.While the rivers that enter Lake Jempang, namely the Ohong river and the Bongan river; and the river that comes out of the lake is the Jantur river.
Measurement of river water discharge using the Velocity Area Method using a Current meter to measure the flow of river water.The width of the river is divided into several sections and each vertical section is measured for depth and current velocity by the three-point method at (0.2, 0.6, and 0.8) of depth.While the calculation of the discharge of each section is used the Mid area method.

River Water Sediment
To determine the sediment concentration, river water samples were taken with the suspended sampler DH-48 at various depths/vertical and horizontal at the same time measuring river discharge.Furthermore, the water samples were analyzed at the Laboratory of Mulawarman University with TSS (Total Suspended Solids) parameters.Sediment loading from the river is obtained by multiplying the total water discharge and sediment concentration, the units of which are adjusted first between the water discharge data and the sediment concentration in the river water.

Land cover of Lake Catchment Area
Data on land cover types for 2000, 2009, and 2018 were obtained from the Ministry of Environment and Forestry.These land cover type data were interpreted from Landsat imagery.In addition, data on administrative boundaries were obtained from data from the Indonesian Geospatial Information Agency (BIG).Spatial land cover analysis was carried out in ArcGIS software using overlying and cross-tabulation analysis in ArcGIS.

River Discharge
Mahakam Cascade Lake consists of Lake Jempang, Lake Semayang and Lake Melintang.During the dry season, the water between Lake Semayang and Lake Melintang is separated, and the water that comes from Lake Melintang enters Lake Semayang through the river channel in the village of Melintang Village.But when the rainy season those two lakes' waters are merged, so Melintang Village becomes in the middle of the lake.At the time of measurement of discharge and sediment carried out in the rainy season, the waters of Lake Semayang merge with Lake Melintang.The rivers that enter Semayang-Melintang Lake are the Kalaha River, the Enggelam River, and the Rebak Rinding River.While the Pela River is the outlet of the lake.
The results of the measurement of river water discharge entering and leaving the cascade lake are presented in Table 1.Based on this table, it shows that the river entering Semayang-Melintang Lake with the largest discharge is the Rebak Rinding River at 129.89 m 3 /sec, then the Kahala River at 105.21 m 3 /s and the Engelam River at 8.95 m 3 /s.
The characteristics of the Rebak Rinding River are different from the Kahala River and the Enggelam River.The water of the Kahala River and the Enggelam River comes from their catchment areas.While the Rebak Rinding River is a river that is directly connected to the upstream Mahakam River, so the watershed coverage is very wide, namely the upstream Mahakam River.
The flow of the Pela River is influenced by the tides of seawater indirectly through the Mahakam River.When the water level of the Mahakam River is high due to the influence of sea tides, the flow of water from the Pela River is restrained, so that the Pela River discharge becomes small.This happens when the tide is high so that the Mahakam River's water level is blocked.At the time of measurement of the discharge in the Pela River, there was no effect of tidal water and the results of the calculation of the discharge showed a fairly large value of 227.62 m 3 /s.Analysis of the instantaneous discharge based on the results of measurements in the field, in June 2020 shows that the discharge that enters the lake is 244.05 m 3 /s, while that which leaves the lake is 227.62 m 3 /s, increasing water storage by 16.43 m 3 /s.Compared with the discharge measurement in October 2017, the discharge that enters the lake is 284.96m 3 /s, the discharge that leaves the lake is 280.51 m 3 /s, and the water storage (storage) is 4.45 m 3 /s [12].Changes in the amounts of water reservoirs are strongly influenced by the flow conditions of the Mahakam River, both due to the influence of tides and river flows.The upstream Mahakam, considering that the Rebak Rinding River is directly connected to the upstream Mahakam River has a very large watershed area.
At Lake Jempang the rivers measured were the Jantur River (the lake outlet), the Bongan River, and the Ohong River.At the measurement location, the Bongan River has a width of 15 meters and the deepest water depth is 4.1 meters, with a water discharge of 10.31 m 3 /s, and the Ohong River it has a width of 24 meters and the deepest water depth of 3.1 meters, with a water discharge of 19,77 m 3 /s.

River Sediment Loading
The results of sediment measurements in rivers entering and leaving the Mahakam cascade are presented in Table 2. Based on the table, shows that the Rebak Rinding River has a sediment load value of 3,859.66tons/day, the Kahala River 149.4 tons/day, the Enggelam River 4.69 tons/day, and the Pela River 197.32 tons/day.At Lake Semayang-Melintang the sediment load value of the incoming river water has greater value than the river leaving the lake, so it can be said that there has been a sediment trap in the lake.Based on the sediment load value, the Rebak Rinding River is the river with the largest contribution to sedimentation in Lake Melintang-Semayang.The Rebak Rinding River is a river that flows directly from the upstream Mahakam River.The watershed area is very large (about 46.000 km 2 ), when compared to the Kahala watershed (933 km 2 ) and the Enggelam watershed (627 km 2 ).The upstream Mahakam watershed has a wavy to hilly topography, even approaching the mountains.The land use varies greatly: mining, plantations, agriculture, and industrial forest plantations so erosion is quite large.
Based on the results of instantaneous sediment measurements on rivers entering and leaving the lake in June 2020, sedimentation was 3,816 tons/day.When compared with the sedimentation of Lake Melintang-Semayang in October 2017 was 696 tons/day [12], there is a fairly large increase in sedimentation.Although this cannot be directly compared, given the different measurement time conditions and also only from the instantaneous measurement results.But it can be an early indication that sedimentation in the lake needs to get a top priority in the context of the conservation of the lake ecosystem.This sedimentation rate needs attention, considering the shallow waters of the Mahakam cascade lake, during the dry season the condition of the lake is only in the form of river channels that are inundated with water and others in the form of dry soil so the effect of sedimentation on the sustainability of the lake will become more real.
Based on the sediment load the river entering the lake appear to be the Rebak Rinding River which has the largest contribution to sedimentation that occurs in Lake Melintang-Semayang.Sediment load from the Rebak Rinding River can reach about 77% of the total sediment that enters the lake.Therefore, controlling sediment originating from the Rebak Rinding River is the priority in the context of lake conservation.
The results of the analysis of lake water level data during 1989 -2010 show that the longest duration of extreme receding occurred in 1997 (192 days).Meanwhile, the longest extreme flood conditions occurred in 2006 (63 days).Extreme floods are becoming more frequent, especially since 2000, extreme floods occur almost every year.But on the contrary, extreme receding events are also increasingly frequent over a long period.In 2000, 2001, and 2002 extreme receding occurred for 25 days, 26 days, and 15 days respectively.Even more in 2006, the extreme receding lasted even longer (67 days).So when a big flood occurs for a long time and vice versa when there is a drought it also lasts for a long time.
Figure 2 shows that there is a tendency for the lake water level to increase in moderate and extreme flood conditions.But on the contrary, the water level at receding conditions tends to experience a relatively small decrease.Changes in land cover are one of the factors that affect the increase in water level toward flood conditions, especially during rains.Changes in land use from forests to plantations, agriculture, and settlements cause changes in increased flooding [14].

Figure 2. Duration of flood and receding events
An important factor that generates the character of flood-exposed lakes is the fluctuation of the lake's water level.The water level fluctuations play a major role in stimulating high levels of biological productivity in lake waters.The pattern of water level fluctuations of cascade lakes tends to be higher, especially during flood conditions can harm the function of lake waters as a habitat, where large floods wipe out habitat resources.And when the dry period occurs for a long duration, it no longer supports the life cycle order.If the pattern of water level fluctuation is disrupted, it will affect the lake ecosystem [15].Changes in water fluctuations can also change the structure of vegetation that grows in lake waters.The greater the ratio between the area of the lake catchment area and the surface area of the lake, the greater the role of the catchment area.The area of the Cascade Mahakam lake varies depending on the season, during the rainy season the area of the lake waters can reach 776.6 km 2 and the area of the lake catchment area is 51,573 km 2 , so the ratio of the area of the catchment area to the lake waters is 66 times.This condition indicates that the sustainability of lake water is strongly influenced by environmental conditions in the catchment area.
The results of the classification of lake catchment land cover in 2000, 2009, and 2018 are presented in Figure 3.In general, in 2018 land cover was mostly dominated by forest, both on dry land and swamps, as well as plantation forest which reached 34,895 km 2 (67 %), followed by shrubs.both in swamps and non-swamps of 12,103 km 2 (23%); and a combination of open land, plantations, mining, and mixed dry land agriculture with an area of 3,798 km 2 (7.3%).
From 2000 to 2018 there was an area increase in plantation, dry agriculture, and mining (2,432 km 2 , 245 km 2, and 166 km 2 , respectively).A study mentioned that industrial forest plantations/plantation land can increase very high sediment yields [16].On the other hand, mining also increases sediment and decreases water quality [17] and agricultural land has a large share in sedimentation [18].So during this period, there was an increase in land sensitivity to erosion.
On the other hand, during the same period, there was a reduction in forest area (Primary Dryland Forest, Secondary Dryland Forest, Primary Swamp Forest, Secondary Swamp Forest, and Plantation Forest) by 2,063 km 2 (5.6%).The area of residential areas also increased by 42.6 km 2 (3.03%) as can be seen in Figure 4.The physical characteristics of the lake water catchment area, especially land use patterns, affect water yield.Land use conversion can increase changes in flow properties produced by lake catchment areas.Changes in forest land to agriculture/mining in addition to increasing erosion also increase river flow fluctuations, so that lake water level fluctuations are more extreme.
Forests have a function in regulating watershed hydrology, i.e. reducing floods and increasing baseflow [19].Forest has a very high infiltration capacity which often exceeds the intensity of rainfall [20].The hydraulic conductivity of forest soil is also high, compared with agricultural cultivation soil [21].As the result, it can reduce surface runoff, peak flow, and sedimentation.From 2000 to 2018 there has been a decrease in forest area by 2,063 km 2 and conversely, there has been an increase in plantation land of 2,432 km 2 , dry land agriculture 245 km 2 and mining land 166 km 2 , and residential land 42.6 km 2 .
The decrease in forest area and the increase in the area of plantation land and mining that occurred in the lake catchment are the main factors that can change the hydrological process of the watershed and have an impact on the sustainability of the function of the Mahakam cascade lake.Therefore, attention needs to be paid to controlling land use change and aspects of soil and water conservation in agricultural/plantation cultivation.

Conclusion
Water level fluctuation is the main factor forming the characteristic of Mahakam cascade lake.The current condition is the fluctuation pattern becomes more dynamic, mainly in the high magnitude at moderate to extreme flood.The sedimentation in the lake is also high originating from the large sediment yield of the Rebak Rinding sub-watershed.The lake sustainability strongly depends on the condition of the catchment area which reached 66 times larger than lake water bodies.Moreover, during the last 19 years, the forest area was reduced by 2,063 km2 and as a consequence, increased erosion-sensitive area by 2,695 km2.The change in water level dynamic, sedimentation, and land cover in a long term could be threatening the sustainability of the lake ecosystem.The Land Sensitive to Erosion

Table 1 .
Results of measurement of river water discharge entering and leaving the lake

Table 2 .
Sediment load on rivers in and out of the lake Land Cover in The Catchment Area Lake catchment areas have a very important role in the availability of water and lake water quality.