Environmental Challenges

The Black River was chosen for rehabilitation because of its importance as a potential spawning groung for fish in the Shannon. The most significant problems impacting the Black River are:

1. Silting
2. Collapse of soft banks
3. Tunnelling of river by bush and tree overgrowth
4. Domestic sewage

1 - Silting: This is the biggest problem, especially in the western tributary. Silt comes from glacial deposits of sand and gravel that form part of the hills around. Very fine clay of glacial or closely postglacial origin forms the riverbed and in places part of the banks of the western stream over much of its course. This is being actively eroded due to its soft nature, and it contributes a lot of fine material to the sediment. Silt and clay is also contributed by the underlying weathered shales and sandstone which are being eroded in the upper streams. Silt is a natural part of this river, but normally it would be transported along the channel as quickly as it is washed in, leaving gravels clean and oxygenated. However, industrial quarrying and farming, present and past, may wash in more than the river can easily transport. Lack of silt traps in drainage dykes, overgrazed fields, and poorly specified drinking places or crossing points can add huge volumes of silt. When silt makes up a high percentage of stream sediment, insect life is adversely affected because there is no space under the sediment surface for insect larvae to live, and salmon and trout spawning is adversely affected because there is no oxygenated water between pebbles to keep fish eggs alive.

2 - Collapse of soft banks: The stream banks are in places very soft because they are formed of alluvium and clay. The Black River is not of steep gradient or high in energy, but it cuts down and sideways through the soft material, making banks steep and liable to collapse. Collapsing banks cause choking, shallowing and widening of the stream, and collapse of bankside trees and fences. All of this causes an increase of undesirable fines in the river sediment. This can cause land owners to want to dig out the obstruction, generally causing a hiatus in the natural environment, more silt in sediment downstream, giving rise to the same problems again.

3 - Bush and tree tunnelling: If bushes and trees grow densely on both sides of a stream they will meet in the middle, and very little light will get into the stream. This is called tunnelling. Streams need some trees and bushes, to give shade from heat, substrate for insects to live on, and cover from predators. But too much shade is as bad as too much light, and 100% shade means that practically nothing will live in a stream, because there is no weed or algae to start the plant–insect–fish–bird–mammal food chain. What is required is alternating cover and light, cover and light. Some parts of the Black river course are completely wooded over, with no light getting in for much of the growing part of the year.

4 - Sewage: In places of dense habitation (like a village or a housing estate), sewage is usually collected together for treatment, before the harmful waste is removed and transported to a safe permanent disposal site. Bridgetown has two sewage treatment facilities, and both appear to be continuously discharging into the river. Last year there was apparently a failure of sewage treatment plants on the river. In a primary catchment like the Black river, someone downstream will soon be drinking that water. Fish and every other type of organism have to survive in it and people we know are swimming, canoeing, sailing and fishing in it. Waste water needs to be looked at if we are to have a high quality rating for the catchment.