Dam-building to repair and restore natural hydrology

Peat swamps have an important regulatory role as reservoirs of water.

• Reservoirs such as these control the hydrology of entire river catchments.

Peat acts as a ‘sponge’, storing water during the wet season and releasing it slowly into neighbouring rivers during the dry season... 

• thus preventing flooding during periods of high rainfall and maintaining river flow during periods of drought. 

Peat swamps are naturally waterlogged year round; this prevents decomposition, leading to accumulation of organic material, i.e., peat. If they are able to remain waterlogger all year round, this also makes peat swamps very resistant to fire and allows the swamps to act as a natural breeding ground for fish. 

• When peat is drained, however, decomposition starts and the peat becomes very vulnerable to fire. 
  
• Fish that breed in these sites later provide 80% of animal protein for the surrounding communities. 
  

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During a period of eight years between 1997 and 2005, Sabangau was subjected to intense illegal logging. To extract the timber from the forest, thousands of small canals were dug, usually 1-2 m wide and 1 m deep, and varying in length from 1 to 15 km (most are 2-5 km long). Using these canals, cut timber is floated to the main rivers and then to sawmills for processing. The construction of canals for timber extraction seems unique to deep peat-swamp forests. The interface between peat and forest is a delicate equilibrium requiring stable hydrological conditions and a steady influx of plant material. The canals destroy this balance by rapidly draining the top layer of peat and substantially lowering the water table. Short-term effects of drainage include lack of drinking water for forest animals, virtual elimination of fish stocks, increased tree falls and shorter fruiting cycles. More seriously, dried peat burns very easily, as evidenced by major fires within Sabangau between 1997 and 2006, which have burnt 15% of the forest area.

A video of the dam building efforts in Sabangau.

To counter this threat, dams are built in canals by members of the local community, directed by the CIMTROP TSA and Community Patrol Team. We have trialled a number of designs to identify those that are stable, and cost- and time-effective. The selected designs are now being tested in a number of combinations and different spacing frequencies. There are two main designs, single-wall dams, made from either externally-harvested galam (Melaleuca sp.) wood or small saplings; or two-wall dams, made from either externally-harvested galam or long-lasting, durable balangeran (Shorea sp.) wood with peat-filled bags in between, and over planted with locally-sourced shrubs. Another designs uses native species that were good at regenerating themselves, thus creating ‘living dams’. All the dams are angled against the current, with excess water channelled over the centre of the dam to avoid too much water pressure building up during the wet season, and prevent erosion of the peat soil on either side of the dam. The most effective designs appear to be a combination of six dams of different designs spaced 10 m apart over a 50 m stretch, and these ‘kelompoks’, or dam-sets, are the current design of choice. 

During 2010 we built 379 dams on nine canals in the Natural Laboratory, thus closing these canals to future access and retaining water and litterfall in the ecosystem, to restore natural hydrology. We have built 457 dams since the start of this project. By retaining water the peat remains wet, significantly reducing the rate of peat degradation and the risk of fire. By preventing the outflow of leaves and other macro organic material leaving the forest, the canals will start to fill in naturally, leading to a rise in the water table, which is the long-term goal. As well as building dams, we are also monitoring water depth and flow rates each month to assess the impact of different kinds of dams and dam systems. This monitoring has shown these dams to be highly effective at reducing water flow in canals.