How A Dam Lock Works

This article discusses how navigation locks at a dam work, and a bit about locking operations.

Many of the world's long, navigable rivers change elevation during their courses, from beginning to end. Commercial traffic is vital along these rivers, and a barge can't go down (or up) a hill. How do boats change elevation with the river? Usually, through the navigation locks on a dam. These locks, in essence, act as "elevators" to raise or lower a boat to the level of the river it wants to travel.

Dams are interesting structures in and of themselves, with locks being, perhaps, one of their more fascinating features. Locks are an ingenious answer to a common problem. As a bystander watches the water drain in, or flow out, of a lock, he or she may wonder how it all works.

A lock is usually situated at the end of a dam, usually on the other end from the powerhouse and turbines. It is basically a chamber, open at the top, with watertight doors at one end. A person called a lockmaster operates the lock and oversees commercial and recreational traffic. That person makes the decisions about when to lock through boats, and how to control the traffic flow.

For instance, barges still travel on weekends. So do recreational boaters. If many recreational boats want to lock through, and barges are also waiting, the lockmaster must make the decision which go first. Most lockmasters give priority to commercial traffic, since most locks are large enough to accommodate several small craft at one time, and these can all be locked through together, after the barges.

Boaters do not usually pay a fee to lock through a major dam. These are often operated by state or federal agencies and tax dollars pay for the locking operations. Whether recreational or commercial, boats signal their arrival at the lock by radio, or small craft by sounding a horn on the lock. The lock is prepared, the boats enter and tie on to the floating mooring posts inside the lock, along the wall. They are then locked through.

The lock is filled and emptied through gravity. No pumps are required. If the boat is locking downstream, to a lower level, then it enters the filled lock and the downstream valves underneath the lock open and water flows out into the river, emptying the lock, to the downstream river level.

If the boat enters the lock to go upstream, the downstream valves are closed, and the upstream valves are opened, allowing water from the higher level to spill into the lock chamber, filling it. When the water level reaches the upstream level, the gate is opened and the boat travels on upstream. Gravity does all the work. Depending on the size of the lock chamber, the process can take anywhere from 30 to 45 minutes.

Some smaller dams may have more than one lock, and boats must travel up or down, one step at a time, to reach the next river level. The principle is the same, though, as for a single lift-lock dam.

The advent of navigation locks has made rivers with formerly limited navigability and commercial potential into profitable waterways. They are essential components in the world shipping trade.

© High Speed Ventures 2011