The Fish-Sundays River Canal Scheme comprises a canal and tunnel system which supplies Orange River water from the Great Fish River valley to the Sundays River valley to supplement existing water supply in the Eastern Cape. Since 1992 water from the Sundays River valley has been supplied to Port Elizabeth. It is estimated that up to 200 million m3 of Orange River water could eventually be transferred to the Port Elizabeth metropolitan area annually.

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Orange River water is diverted from the Great Fish River by a weir at Elandsdrift into an aqueduct which winds approximately 65 km along steep slopes and cuts through the Bosberg chain between Cookhouse and Somerset East. The main feature of this aqueduct is the 13,1 km Cookhouse Tunnel through the Bosberg, which was completed in 1978. The canal discharges into the Little Fish River near Somerset East via a multi-stepped chute, from where the water flows down the Little Fish River for some 40 km to the De Mistkraal Weir.

The droughts of the past have created a critical situation in the Darlington Dam (formerly Lake Mentz) region which, despite being a fertile area, requires an assured water supply. The serious drought of 1966 and 1967 emphasized the necessity to commence work on the Skoenmakers Canal with a capacity of 22 m3/s to link the Great Fish River to Darlington Dam as soon as possible. In view of an expected increase in irrigation below Darlington Dam and the demand for water in the Port Elizabeth metropolitan area, it was decided to replace the Wellington Grove pumping station with De Mistkraal Weir upstream of Wellington Grove and a short section of connecting canal to the beginning of the Skoenmakers Canal.

With the completion of the De Mistkraal Weir in 1987 the possibility of transferring water to Darlington Dam at the full design capacity of the Skoenmakers Canal was created. This water dilutes the salinated water of Darlington Dam and therefore presents an immediate benefit by improving the water quality for citrus farming in the lower Sundays River valley. A long-term economic benefit of the weir is the development of irrigation potential in the lower Sundays River valley. As a result of the developments, a further 16 500 ha can now be utilized for citrus farming.

A further extension of the scheme was launched in 1989 in order to provide water to Port Elizabeth where supplies were limited due to a severe drought. Water from Darlington Dam and the Sundays River irrigation canals flows to the Scheepersvlakte Dam, the main balancing dam for the irrigation scheme. From there water is conveyed by means of a gravity pipeline to a point on the right bank of the Sundays River where a purification works has been constructed. From the purification works, the water is pumped to a balancing dam on the plateau which separates the Port Elizabeth metropolitan area from the Sundays River, from where it flows to the municipality's existing reservoir at Motherwell. Apart from the increase in available water to the Port Elizabeth-Uitenhage metropolitan area, the municipality is less dependent on the Kouga Dam (formerly Paul Sauer Dam). This is to the advantage of the irrigators in the Gamtoos Government Water Scheme as it will increase the assurance of supply for irrigation.

The lower Fish River Scheme was initiated in 1985 and completed in 1992. The purpose of this scheme is to provide sufficient water of a suitable quality to irrigation developments along the Great Fish River in the vicinity of Committees Drift. The scheme consists of the Hermanuskraal Weir in the Great Fish River with a tunnel to discharge flood water and water released from the Orange River into an off-channel storage dam, the Glen Melville Dam in the Ecca River. The distribution system consists of a canal and pipelines to the irrigation areas on both sides of the river. The scheme will enable further irrigation expansion and will ensure that water of an acceptable quality is supplied.

The scheme also makes provision for Grahamstown's increasing requirements. When the scheme was started, Grahamstown was already experiencing problems in meeting its growing demand for water and the Fish River was the obvious source to serve as a supplement. The scheme ensures that a stable supply of good quality water is available, which can be linked to the municipal water supply network. The scheme also benefits rural communities in the Great Fish River catchment (including portions of the former Ciskei) by improving the quality and availability of water.