Dams and Levees
Updated September 9, 2024
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There are six dams that form the Highland Lakes: Buchanan Dam, Inks Dam, Wirtz Dam, Starcke Dam, Mansfield Dam, and Tom Miller Dam. These impound, in order, Lake Buchanan, Inks Lake, Lake LBJ, Lake Marble Falls, Lake Travis, and Lake Austin. The seventh dam in order along the Colorado River is Longhorn Dam, which impounds Lady Bird Lake.
A map of the dams and lakes along the Colorado River. From the LCRA: https://www.lcra.org/water/floods/
Managing Floods in Flash Flood Alley (LCRA)
The History of the Texas Highland Lake Dams (KVUE) LCRA Historic Moments (video series) (LCRA) Visiting our local dams:
Many of these dams used to be open to the public, to walk across or at least walk to, but in the years following the Sept. 11 2001 attacks, the dams were closed to visitors for security reasons. Most are easily accessible from the water, but if you want to see them from land, here are a few suggested sports, that are either free, or very inexpensive
Part of a hydroelectric turbine on display near Mansfield Dam.
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The Seven Dams along the Colorado River in Central Texas Dams
Buchanan Dam (Lake Buchanan)
Roy Inks Dam (Inks Lake)
Alvin Wirtz Dam (Lake LBJ)
Max Starcke Dam (Lake Marble Falls)
Mansfield Dam (Lake Travis)
Tom Miller Dam (Lake Austin)
Longhorn Dam (Lady Bird Lake)
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Dams, Levees, and Dikes are structures that constrain or control the movement of water. Many early civilizations were established along rivers, which supplied water for human use and for irrigation. But fertile rivers “feed” the nearby land with nutrients during periodic flooding episodes. By controlling the extent of flooding (often via levees) and changing the direction and distribution of the waters (via dams and channels) to water crops and supply water to the developing towns and cities, people could take control over the flow of rivers, increasing their value and decreasing their destructive potential.
Here in Central Texas, dams are an integral part of our landscapes and our relationship with the Colorado River. The Highland Lakes are the result of a series of dams, built primarily for flood control, but also to make reservoirs of water for cities and to produce hydroelectric power. Nearly every large “lake” in Texas is the result of a dam, as opposed to natural formation. However, don’t fall for the oft-repeated myth that there is only one “natural” lake in Texas (Caddo Lake): first, there are many (smallish) natural lakes in Texas (head down to Palmetto State Park to see one oxbow lake), and second, Caddo Lake may have initially formed naturally (by a massive logjam dam), but it is currently a lake due to human intervention (more on Caddo Lake later).
Austin is flanked by two dams, the Tom Miller Dam to the west and the Longhorn Dam to the east. The Tom Miller Dam impounds Lake Austin, over which the Pennybacker Bridge (360 Bridge) spans in a graceful arch. The Tom Miller Dam is just the latest of several dams built at that site over time, the first in the 1890s (which powered Austin’s moon towers) which failed during the flood of 1900, the second partially completed in 1912, but destroyed in the floods of 1915. The current dam was completed in 1939. At the other end of town, Longhorn Dam, completed in 1960, was built at a traditional ford across the river. The dam impounds what is now called Lady Bird Lake, covered with SUP boards, kayaks, canoes, and rowing shells, as well as the occasional dragon boat. Both Lake Austin and Lady Bird Lake are constant level lakes, their relative depth controlled by the release or impoundment of water by the dams. 
An undershot water wheel in Arkansas, used to power a grain mill
There are natural dams, which we will explore shortly, but more often when we think of dams, we think of large, manmade structures. Dams are built for many different reasons (and often for several at a time). A dam may capture water in reservoirs (impoundment) for human use, for livestock and irrigation, or to control floods by managing the flow rate of a river. Dams may redirect water, for irrigation, mechanical power (water wheels), or hydroelectric power production. Dams may be used to constrain the movement of wastewater or polluted waters, or to hold back waters while building bridges, tunnels, roads, or even dams.
There are other structures similar to dams, but with different names depending upon their location in relation to the river and their purpose. Dams are built perpendicular to the flow of water. Levees are built parallel to rivers, to reduce the chances of flooding by raising the height of the banks (in our area, there are a few in San Antonio, but we usually think of the dikes in the Houston area, where flooding is an all too common occurrence). Dikes are similar to levees, but are built to keep water out of areas that are naturally wet (think of the many dikes in Holland, or travel to Lake Somerville to see a 4715 foot dike along the spillway. Like dams, levees and dikes can form naturally, but we generally focus on those that are either reinforced or constructed by people.
Building a Dam
The first thing to consider when building a dam is its purpose. This will help define where (and whether) to build the dam, and what type, and what agency to request a permit from. Next, there will need to be a site assessment, looking at the geology of the area (a geotechnical investigation) that will look at things like the substrate, rock layers, likelihood of earthquakes, and other geological dynamics, a hydrological analysis, which will assess things like the average rate of flow, seasonal variation, propensity to flooding, how stable is the river within its banks, and what the local sedimentation rate may be. In addition to the physical characteristics of the site, there is an environmental assessment, considering the impact on fish and wildlife and water quality, and seeking ways to minimize that impact through site selection or specific dam features. Once these are fully considered, a more detailed plan can be made for the dam.
In addition to the physical characteristics of the site, and the environmental assessment, dam engineers must consider safety in designing a dam and planning the construction process. While a dam may need to be strong enough to block the movement of water, it also needs mechanisms to allow the flow of water to pass the dam, either during normal flow or during extreme events. The simplest method is to have the water flow over the top of a dam, but this in uncontrolled. Dams often have gated spillways that allow the operator to control how much or how little water flows through the dam. Some of the Highland Lakes are “constant level” lakes, while others are not. The constant level lakes are kept that way by controlling flow in and out of the lake via the gated spillways. In addition, dams need to have emergency or auxiliary spillways, which allow water to pass freely over or around part of the dam in flood emergencies. Dam builders and operators must establish emergency action plans, to prepare for unexpected events. For safety they need to conduct regular dam inspections. In Texas, the Texas Commission on Environmental Quality runs a Dam Safety Program, which oversees all private and public dams in the state. Once these plans are in place, work can begin on the dam, often first by temporarily diverting the river around the dam site via canals or tunnels, or building a coffer dam to separate the dam construction site from the flow of water.
Major Dams near Austin, Texas
Sources: https://www.lcra.org/water/dams-and-lakes/, https://www.twdb.texas.gov/surfacewater/surveys/completed/list/index.asp, https://www.twdb.texas.gov/surfacewater/rivers/river_basins/colorado/index.asp
Additional information about our local Colorado River dams:
Dams in the Bible, In History, and in Warfare
Dams and other river-control methods have been part of historic warfare for centuries. In Daniel chapter 5 we see a very brief retelling of the fall of Babylon to the Medes and Persians. While Daniel focuses on the message on the wall, we read from Herodotus that the way Cyrus finally breached the walls of Babylon was by diverting the river which flowed under the city walls, thus lowering the level enough to allow his soldiers to breach the unsuspecting and overconfident city defenders. According to the Roman author Frontinus, Lucius Metellus diverted a river to flood out his opponent in Spain, which was camped in a low-lying area. Han Xin used soldiers and sandbags to temporarily dam the Wei river, then attacked across the river, only to retreat. When his opponent followed, he ordered the sandbags removed, thus isolating the enemy between the river and Han Xin’s force. In the Bible, we see other examples, whether of divine intervention or perhaps of human ingenuity, where rivers are stopped or flooded areas taken advantage of, including Moses’ crossing of the Red Sea, Joshua’s entry into Canaan across the Jordan River, and Deborah’s luring of Sisera's chariots into the swamps.
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Natural Dams
Not all dams are manmade. Beavers build dams to create areas of deeper water as protection against predators. A beaver dam begins with a collection of logs and sticks that the beaver fells and drags into place. Smaller sticks are interwoven into the dam to add strength. Beavers will collect and place rocks near the base of the upstream face of the dam for strength, then dredge mud and silt from the bottom to seal the dam. They will build their lodge in the deeper waters made by the dammed stream. Beaver dams change the environment, flooding some areas of a creek or river, altering the flow of the river, but they also help filter the water that flows through an area, with the dam slowing water flow, allowing sediments to drop out.
Nature has other ways of building dams without human assistance. It is not uncommon after a flood to see small dams made of log jams and brush clogging up some of our Central Texas creeks. One infamous log jam some 100-150 miles long, known as the“Great Raft,” created Caddo Lake in east Texas. As the Red River backed up, the lake expanded and water levels rose. By the 1830s and 1840s, the lake was already serving as a major route for commerce and riverboat travel, and there were early efforts to begin removing trees from the Great Raif. In the 1870s, the effort was renewed, this time using explosives. In 1873 the final sections of the Great Raft were removed, but the loss of the natural dam had a significant impact on the lake and bayous. In the 1910s, with the discovery of oil beneath the lake, early dam construction began to raise the lake level to facilitate the first overwater oil drilling in the world. Between 1968-1971, the dam was rebuilt, and the current Caddo Dam is used to maintain navigation and manage the level of the lake.
Natural dams can also be formed by landslides and rock slides, and by ice jams. Heavy rain may lead to mud, soil, or rocks sliding down a valley wall into the river below, damming its progress. While we rarely encounter ice here, ice jams can form in rivers during the spring breakup, contributing to floods, but also to damage to structures caused by the pressure of the moving ice. Historically (and even today) as glaciers thaw, ice may break off and clog a glacial stream, creating a lake. When the ice dam breaks, it can trigger widespread flooding, something uncovered by Geologist J. Harlen Bretz in his early 1900s study of the scablands in Washington State.
Benefits and Problems of Dams
While manmade dams can be beneficial (from flood control to irrigation to urban water supply and sanitation), dams can also cause problems. Dams can restrict the movement of fish and other animals, changing their reproductive patterns and ecosystems. They may inundate lands that are already in use, and can become depositories for silt and pollutants. If they are not maintained, they risk catastrophic failures and can flood areas, rapidly increase erosion, and cause death and destruction. When considering whether to build a dam (or even whether to remove one), both the benefits and risks of the dam must be considered. Design features can also reduce the impact on the local ecosystem.
One feature of some dams is a fish ladder or other physical or mechanical device designed to assist fish in moving upstream past the dam. This is particularly important for fish that spawn near the headwaters of a river, but live their life in other parts of the river, or even in the ocean (such as salmon). A particularly unique idea is the “salmon cannon,” a system of pressurized tubes that can move fish rapidly over and around obstacles. Types of Dams
If we think back to the beaver dam, we see nature setting the example for many manmade dams. Historically, people have constructed dams from wood, clay, rock, and soil. Modern dams are often made of concrete (along with rock and dirt), and some are even built of steel (particularly dam gates). The choice of construction material is often determined by the purpose of the dam, and the anticipated water pressure and flow rate. Earthen or embankment dams are the simplest, and often used in areas with low or intermittent flow rates (such as capturing runoff to fill a pond). Smaller embankments, known as saddle dams, may be built along low areas around a planned pond or reservoir to raise the height of the banks.
More complex embankment dams will have an impermeable core, made of rock and gravel, that restricts the flow of water. On the downstream side, there may be a rock toe, an area of heavy rock that allows for some controlled seepage from the dam to flow out without causing rapid erosion, while also strengthening the base of the dam. To avoid water moving under the dam and eroding it away, a cutoff trench may be dug down well into an underlying impermeable layer, and then backfilled to block the flow of water under the dam. Embankment dams may also be reinforced with concrete for added strength and to reduce permeability. Notice that earthen or embankment dams are susceptible to water erosion, particularly from inside, as the water will move through the soil, even if very slowly. When the water in a dam begins to erode internal channels, the process is referred to as piping, and a dam with piping issues is susceptible to failure. Longhorn Dam in Austin is an earth fill embankment dam, measuring 760 feet long. This type of dam uses a pile of dirt and rock, strengthened with concrete. The dam that created Lake Somerville is also an earth fill embankment dam. The Great Northern Dam on Shoal Creek is another embankment dam. Earthen and earth fill embankment dams are generally held in place by friction and gravity - their shape (wider at the bottom) and heavy materials withstand the pressure of water pushing against them. Many concrete dams are also considered gravity dams. Rather than being secured to the bedrock beneath, they use the weight of the concrete to resist the rivers’ movement. Even so, many are tied into the rock on either side for added strength. Mansfield Dam on Lake Travis is an earth fill gravity dam, as is Starcke Dam on Lake Marble Falls. Gravity and embankment dams may be thick and bulky. To allow for thinner, taller dams, other elements or shapes may be employed. A dam may have added supports on the downstream side to withstand the force of water. Such dams are called buttress dams. The Tom Miller Dam at Lake Austin has a slab and buttress section, and Buchanan dam also uses buttresses for strength. Arch dams are curved, with the convex side facing into the flow of water, and take advantage of the natural strength of an arch (a common form also used in bridge construction). Large dams, like the Hoover Dam, are arch dams. Buchanan Dam is one of the largest multiple arch dams in the world, measuring 145.5 feet high and 10,978 feet long. As you may have noticed, Buchanan Dam is mentioned twice, as both a buttress dam and an arch dam. Many dams use multiple construction styles, making classification a bit more complex.
Dams may also be characterized by their purpose or the way they interact with a river. Diversion dams are designed not to impound water, but to change its direction (or at least part of its flow), often for irrigation and other agricultural uses. In Texas, there are several diversion dams along the Rio Grande and the lower reaches of the Colorado River. These dams distribute water to Mexico or the US for agricultural use, or serve as ways to manage flood control. Some of the earliest in Texas were built in the 1700s.
Run of the River Dams also generally allow the water to pass relatively unimpeded (without creating lakes or reservoirs). Some hydroelectric power plants use run of the river dams to channel the water through the turbines, relying on the natural river flow rate, rather than the controlled flow rate from a reservoir. 
Low-head dams may be considered run of the river dams as well. They are designed to allow the water to flow over their crest, and primarily serve to slow water flow and thus reduce flooding. Low-head dams are frequently found on our local creeks, but they can present a particular hazard to swimmers and boaters, as the downstream side can create undercurrents that draw swimmers and boats back into the dam and underwater, leading to drownings. From upstream, they are hard to see, making it even more dangerous for those not familiar with the location of such dams.
One unique type of dam is a lock. On some rivers, either natural rapids or falls, or manmade dams, impede the movement of ships and boats up or down the river, or a canal may be dug across land, changing in elevation as it goes. Locks are enclosed channels with gates at either end. The gates will be closed at the far end as a ship approaches, and the ship floats unto the lock. Then the other gates are closed, and the water level in the lock is adjusted (either raised or lowered) to match the next section of the river. Finally the second set of gates are opened, and the shop sails out. The Panama Canal has several locks to allow ships to cross the land. In Texas, the Colorado River Locks near Matagorda are the only major ship locks I the state. The lock is 1200 feet long, and 75 feet wide, and facilitates recreation and commercial ship and barge traffic. The Waterways Council has a map that shows all the locks and canals in Texas.
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All images by R. Baker unless otherwise noted.
Dams and Levees
1. What is a dam? Briefly describe what it does and how it is different from a levee.
2. Explain how beavers build dams and why. How do beaver dams affect the environment? How else can dams be created in nature?
3. What are the benefits of dams? What are some potentially negative aspects of dams?
4. List five materials historically used in constructing a dam embankment.
5. From pictures, identify the following structures and explain their features and application:
a. Arch dam. b. Buttress dam. c. Gravity dam. d. Diversion dam. e. Hydroelectric dam.
f. Levee or dike. g. Locks. h. Low-head dam (weir). i. Run-of-the-river dam. j. Saddle dam
6. Know the following terms to be able to explain it to someone unfamiliar with dams and levees:
a. Cut-off trench. b. Fish passage/ladder. c. Rock toe
d. Spillway. e. Piping
7. Explain why someone should consider a low-head dam dangerous and avoid it while canoeing, kayaking or swimming.
8. Dams are complicated structures. How do the following issues affect the consideration, design or construction of a dam?
a. Wildlife. b. Water quality. c. Emergency spillway. d. Emergency action plan
e. Safety. f. Geotechnical investigations. g. Discharge structure. h. Slope stability
i. Hydrology and hydrologic analysis. j. Flow net analysis. k. Permitting
9. Research or interview an engineer or other expert on dam safety. Have the expert explain the inspection techniques that they use to identify a failing dam.
10. Do one of the following:
a. Research and/or visit a fish passage/ladder for a dam structure and design/sketch one for a dam near you.
b. Research and/or visit a locks system. Using materials of your choice, construct a model of a lock system with moving parts.
c. Using contour maps, design a dam for a local stream and do/answer the following:
i. Draw a plan view and cross section(s) of your dam showing spillway, emergency spillway, crest, slope angles, toe of slopes, normal high-water elevation and other pertinent features.
ii. What type of dam did you select and why?
iii. What is the Hydraulic Head of your dam?
iv. Outline the limits of the watershed that is tributary to your dam.
v. How large (surface area and volume) of a reservoir will be impounded by your dam?
11. Read Daniel 5 and research how Darius the Mede captured Babylon. Find two other places in the Bible where dams or the control of water are mentioned.
12. Share a devotional with your Pathfinder club on a spiritual lesson that you learned while studying this honor.
1. What is a dam? Briefly describe what it does and how it is different from a levee.
2. Explain how beavers build dams and why. How do beaver dams affect the environment? How else can dams be created in nature?
3. What are the benefits of dams? What are some potentially negative aspects of dams?
4. List five materials historically used in constructing a dam embankment.
5. From pictures, identify the following structures and explain their features and application:
a. Arch dam. b. Buttress dam. c. Gravity dam. d. Diversion dam. e. Hydroelectric dam.
f. Levee or dike. g. Locks. h. Low-head dam (weir). i. Run-of-the-river dam. j. Saddle dam
6. Know the following terms to be able to explain it to someone unfamiliar with dams and levees:
a. Cut-off trench. b. Fish passage/ladder. c. Rock toe
d. Spillway. e. Piping
7. Explain why someone should consider a low-head dam dangerous and avoid it while canoeing, kayaking or swimming.
8. Dams are complicated structures. How do the following issues affect the consideration, design or construction of a dam?
a. Wildlife. b. Water quality. c. Emergency spillway. d. Emergency action plan
e. Safety. f. Geotechnical investigations. g. Discharge structure. h. Slope stability
i. Hydrology and hydrologic analysis. j. Flow net analysis. k. Permitting
9. Research or interview an engineer or other expert on dam safety. Have the expert explain the inspection techniques that they use to identify a failing dam.
10. Do one of the following:
a. Research and/or visit a fish passage/ladder for a dam structure and design/sketch one for a dam near you.
b. Research and/or visit a locks system. Using materials of your choice, construct a model of a lock system with moving parts.
c. Using contour maps, design a dam for a local stream and do/answer the following:
i. Draw a plan view and cross section(s) of your dam showing spillway, emergency spillway, crest, slope angles, toe of slopes, normal high-water elevation and other pertinent features.
ii. What type of dam did you select and why?
iii. What is the Hydraulic Head of your dam?
iv. Outline the limits of the watershed that is tributary to your dam.
v. How large (surface area and volume) of a reservoir will be impounded by your dam?
11. Read Daniel 5 and research how Darius the Mede captured Babylon. Find two other places in the Bible where dams or the control of water are mentioned.
12. Share a devotional with your Pathfinder club on a spiritual lesson that you learned while studying this honor.