Design and operation of Locks and Weirs
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Locks and Weirs

Weirs 1 - 10 are discussed together as they have similar features and functions. South Australia constructed Locks 1-9 and NSW constructed weir 10. This relationship is preserved with SA operating weirs 1-9 and NSW operating weir 10 for the Commission.

The River Murray is permanently navigable to the top of the Mildura Weir pool, near Nangiloc, a distance of 970 kilometres from the mouth. Eleven weirs with locks, each raising the water level behind it by an average of 3.1 metres, create a continuous series of stepped pools from Blanchetown to Mildura. The Murray upstream of Nangiloc is only navigable during periods of high flow.

Navigation was the first commercial use of the Murray-Darling River system. Steamships commenced operation in 1853 and a prosperous river trade quickly developed. Irrigation from the River Murray, which commenced in Victoria in the 1870s, conflicted with navigation as during dry periods the irrigators continued to take water which resulted in very low river levels which restricted navigation. South Australia benefited greatly from river trade and regarded irrigation as a threat.

The first lock and weir on the Murray was completed in 1922 at Blanchetown in South Australia and the last was completed at Euston, near Robinvale, in 1937. The small locks on the barrages at the Murray mouth were completed in 1940. The original Torrumbarry weir, downstream of Echuca, was replaced with a new structure in 1996.

The locks were generally built too late to be of benefit to the river trade as the development of railways and the growth of motorised road haulage had overtaken river transport. The resulting decline in the river trade led to the abandonment, in 1934, of the construction of further weirs and locks purely for navigation purposes. After that time they would only be constructed if the weir also assisted irrigation. Locks 12 to 14 which were to have been constructed between Mildura and Euston, and 16 to 25 between Euston and Torrumbarry, were abandoned.

Today there are 13 navigation locks on River Murray weirs and two locks on the barrages at the Murray mouth. They are maintained and operated to serve an increasing number of houseboats, tourist cruise boats and other recreational craft. The locks and weirs on the Murray make an enormous contribution to the tourist industry in South Australia and the Sunraysia Region.

Lock Sizes

There are two sizes in the locks on the Murray (not including the small locks on the barrages). Those upstream of the Darling junction are shorter than those between Blanchetown and Wentworth. The River Murray Commission adopted the larger lock below the Darling junction because of the greater volume of traffic along that section of the river in the early 1920s.

Following discussions and correspondence with people involved in navigation on the Murray in 1912, the Engineer-in-Chief of South Australia, Mr Graham Stewart, instructed Captain Johnston on the approximate size requirements for locks on the River. The locks were to be sufficient 'to accommodate one of the large steamers now in use, two of the large barges, probably abreast, at one lockage'. Alternatively if that size proved too costly 'a lock to pass vessels singly, but capable of being enlarged in the future, if necessary'.

• a large lock, 56 feet by 275 feet (17 by 83 metres); and
• the smaller lock, 56 feet by 170 feet (17 by 51.5 metres).

The overall dimensions of the lock chambers on the Murray are slightly larger than those quoted by Johnston.

As shown in the table below, weirs 1 to 10 were constructed between 1922 and 1935. Each weir has a lock constructed next to it, to facilitate navigation.

Purpose

The purpose of weirs 1 to 10 (and the associated locks) is to provide:

• permanent navigation between the Murray Mouth and Wentworth;
• relatively constant pool level to facilitate pumping for irrigation and water supply; and
• Weir 9 also raises the water level high enough to allow gravity diversion to Lake Victoria.

Water is pumped for both irrigation and for diversion to major urban and industrial centres including Adelaide, and the 'Iron Triangle'.

The Catchment

The catchment upstream of weirs 1-10 is most of the Murray-Darling Basin. The major tributaries (including the Darling, Murrumbidgee, Kiewa, Ovens, Goulburn and Mitta Mitta rivers) all join the River Murray upstream of weir 10.