What is a Penstock?

Mary McMahon

A penstock is a channel used to feed or carry away water. The flow of the water through the penstock can be controlled with a sluice or gate that is raised and lowered. This term, and the technology, originate in water mills that used the flow of water to rotate a wheel that in turn generated energy for milling grains. Similar technology can be seen in use in hydroelectric dams, as well as flood control systems that are designed to trap and regulate the flow of water to prevent sudden changes in water level.

A penstock is used to feed or carry away water.
A penstock is used to feed or carry away water.

Penstocks can take the form of pipes or long channels, depending on the facility. Historically, channels were most commonly used and they were dug to connect with an existing waterway. When the sluice is fully open, water flows freely through the penstock. When it is closed, the water is limited and less water enters. A grate or filter may be used to trap large debris like branches and other floating materials so that the debris does not enter the channel and clog it.

A penstock is designed to regulate the flow of water that is released.
A penstock is designed to regulate the flow of water that is released.

Controlling the amount of water in the penstock allows people to regulate how much water is released at the other end. For things like flood control, penstocks can be wide open in the dry season to allow water through, and they can be partially closed during wet times of the year to hold water back. A containment tank or pond may be located behind the penstock to hold water that would otherwise build up and put pressure on the sluice.

Being able to completely close the gate or sluice allows people to drain penstocks so they can be inspected, serviced, and repaired. These channels usually need regular cleaning to remove debris that accumulates over time and to scrub down the sides to clear away algae and bacterial mats. Holes, cracks, and other problems may develop and require periodic repair. Routine maintenance is used to prevent catastrophic failures that might cause flooding and other problems.

In addition to being used to direct the flow of water into or through something, a penstock can also be used to control outflows. A common example can be seen at landfills and sewage treatment plants. Fluids from both facilities need to be released but the rate of release must be controlled for safety. In the event of contamination, the ability to close a gate and completely cut off the release of water is also important for protecting the surrounding environment.

The flow of the water through the penstock can be controlled with a sluice or gate that is raised and lowered.
The flow of the water through the penstock can be controlled with a sluice or gate that is raised and lowered.

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Discussion Comments


I never will forget the time one of my neighbors was killed. His killer admitted to throwing his body in a nearby reservoir.

The man threw him in an area near the penstock, so the authorities had to drain and drag the penstock. They found his body there, tangled among limbs and fishing line.

I’ve never been able to think of that reservoir the same again. Knowing what the fish could be feeding on in the penstock made me not want to fish there anymore.


Some towns could become completely destroyed by flood water if the dam on a large reservoir were to break. I live in one of these low lying areas, and I’m always nervous about the dam and penstock malfunctioning somehow.

A large culvert transports water through the giant dam and into the channel on the other side. When the gate on the penstock is open wide, the rushing water creates rapids, and it’s an intimidating thing to see.

I once read in a newspaper article that if explosives or an earthquake were to destroy the dam, my town would be wiped out. Thankfully, I don’t live near a fault line, and security around the dam reduces the threat of a bomb.


I live about an hour away from one of the largest man made lakes in the world. There is a large dam between the upper and lower areas of the lake, and it is an extremely important tool for flood control.

In spring, the sluice is partially closed, and water levels of the lower lake are high. You can’t go to the beaches during the spring, because they are under water.

In the fall and winter, the sluice is open, so the lower lake’s swimming beaches, which are closed for the season, take on more water. This is a great time to visit the upper lake’s sandy areas, because they are greatly enlarged.


I saw the penstock at the dam on a local lake being cleaned out this week. I really never knew that they did that. Every time I have been there, water has been rushing through at a rapid rate, creating swirls and foam.

The channel leads to the lower lake, and lots of people fish in it. I suppose if large branches were accumulating there, they could break people’s fishing lines if caught on them.

About ten workers were in the penstock, and one of them had a backhoe. He was lifting debris out of the small amount of water left in the channel. I could see lots of wet weeds and limbs, so it was probably time for a good cleaning.


There used to be a mill in the small town where I grew up. It has not been a working mill for many years now, but you can still visit the area and get a good idea of how it all worked.

I am familiar with this mill because it is right on the river and we would go there often in the winter to ice skate.

As a kid I wasn't too interested in how the mill worked,the penstock gate or any of the other pieces of equipment that were part of keeping the mill running smoothly, but now I am quite fascinated by it all.

Every time I make a trip back home, I like to stop by the area where the mill was and picture what it was like when it was in operation.

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