Chris Torr, of Fielding, asks :-

When a river is dammed will the lake behind the dam eventially fill with, material eroding from the mountains and riverbed upstream?

Alex Sutherland, a retired civil engineer and former Dean of the Faculty of Engineering at the University of Canterbury, responded.

When a river is dammed the resultant reservoir starts to fill with the coarse sediment (gravel and sand) carried by the river. Sediment deposition occurs first at the head of the lake and slowly extends down towards the dam. As it does so the volume available for water storage in the lake is reduced but the maximum lake level is unchanged. This limits the flexibility for power generation but does not affect the maximum power output significantly.

There are many examples throughout the world of reservoirs that have completely filled with sediment. New Zealand’s best example is the Waihopai Dam in Marlborough which filled with sediment in only a few tens of years. This eliminated the ability of the reservoir to store water in times of high river flow and to release it in times of low flow. The power station is now “run-of-the-river” meaning power generation is dictated primarily by flow in the river and not by customer demand.

It is sometimes suggested that sediment in reservoirs could be removed by mechanical means. There are two problems with this. First the cost of doing so would almost inevitably be excessive. Secondly to be effective very large quantities would need to be removed and then what does one do with the sediment?

The time required for a reservoir to fill can be estimated if there is data available on the rate at which sediment is carried by the river. This calculation was done for Roxburgh Dam on the Clutha River. The data turned out to be inadequate and the reservoir started to fill more rapidly than expected. This was not sufficient to affect power generation but did cause increased flood levels in Alexandra.

An attempt was made to sluice sediment from the reservoir through gates in the base of the dam. This was only partially successful. However most, but not all, sedimentation in Lake Roxburgh stopped when the Clyde Dam was built upstream. Lake Dunstan, behind Clyde Dam, is now showing signs of sedimentation in the Kawarau Arm at Cromwell.