Myth Busters: The Dam Earthcache EarthCache

Dam: a barrier constructed to hold back water and raise its level, forming a reservoir used to generate electricity or as a water supply.

Man-made dams are structures built across rivers to regulate water flow, create reservoirs, generate hydroelectric power, and control flooding. Geologically, dams alter the natural flow of a river by slowing down or stopping the movement of water, which can lead to sediment buildup behind the dam and increased erosion downstream due to changes in water velocity. Over time, this can reshape the riverbed and banks, as slower water upstream encourages sediment deposition while faster-moving water downstream can strip away soil and rock, deepening channels and increasing bank instability. Additionally, dams create uneven flow patterns, as water tends to be released more forcefully in certain areas depending on the dam’s structure and the volume of water passing through. This variation in flow rate can increase erosion at specific points along the riverbanks, especially where water exits with greater force.

A common belief is that “larger dams always create more consistent and controlled water flow, leading to less erosion.” This assumption suggests that a larger structure inherently distributes water more evenly and minimizes the erosive power of flowing water. However, the relationship between dam size, flow rate, and erosion is complex, and smaller dams with uneven flow patterns at the ends may actually increase localized erosion. 

For this earthcache, we will explore whether dam size truly leads to more uniform flow and reduced erosion or if other factors, such as dam design and water pressure, play a larger role.

The posted coords have dam #1, and the reference waypoint about a mile upstream is dam #2. You must visit both dams to compare and contrast to enlighten yourself about this myth.

Myth Busting: Do Larger Dams have more consistent waterflow and therefore less erosion?

Dams have a significant impact on river systems, altering both the physical and ecological characteristics of the waterway. They affect the flow of water, the transport and deposition of sediments, and the stability of riverbanks. To understand how dams influence rivers, it’s important to examine two key factors — erosion and flow rate — and how they interact.

1. Erosion from Dams

Erosion is the process by which soil, rock, and other materials are worn away and transported by natural forces such as water flow. Dams affect erosion in multiple ways:
    •    Upstream Sediment Deposition:
When water flow slows as it approaches a dam, the reduced velocity causes sediment (sand, silt, gravel) to settle and accumulate behind the dam. This creates a sediment trap, which over time can fill up reservoirs and reduce the dam’s capacity to store water.
    •    Downstream Sediment Starvation and Increased Erosion:
Since sediment is deposited upstream, the water released from the dam downstream is often sediment-poor, or “hungry.” This sediment-starved water has increased erosive power because it naturally tries to pick up sediment from the riverbed and banks to regain its sediment load equilibrium. This can result in:
    •    Deepening of the river channel downstream (bed erosion)
    •    Undercutting and collapse of riverbanks (bank erosion)
    •    Increased transport of gravel and larger particles downstream
    •    Localized Erosion at Spillways and Outlets:
The areas directly downstream of a dam’s spillway or outlet often experience intense erosion due to the concentrated, high-velocity water discharge. The force of water hitting the riverbed can create plunge pools, scouring the bedrock or sediment beneath it and causing instability in the riverbanks nearby.

2. Flow Rate and Dams

Flow rate refers to the volume of water passing through a river or dam over a given period of time, usually measured in cubic meters per second (m⊃3;/s) or cubic feet per second (cfs). Dams regulate flow rate by controlling how much water is released and when. This regulation affects the river’s natural hydrology in several ways:
    •    Reduction in Natural Flow Variability:
Rivers naturally experience seasonal changes in flow rate due to rainfall, snowmelt, and other weather patterns. Dams disrupt this variability by creating more stable and consistent flow conditions. This can reduce the river’s ability to transport sediment and reshape its channel naturally.
    •    Increased Peak Flows During Releases:
When a dam releases water, especially during flood control or power generation operations, it can create short-term surges in flow rate. These sudden increases in velocity can erode the riverbed and banks downstream more aggressively than natural flow patterns.
    •    Uneven Flow Distribution:
Dams with uneven outlet structures or spillways may release water more forcefully at certain points, creating inconsistent flow patterns. For example, a smaller dam may have higher flow rates at the edges compared to the middle, which leads to uneven pressure on the banks and increased localized erosion.

3. How Erosion and Flow Rate Are Connected

Flow rate and erosion are directly linked — the faster water moves, the greater its ability to pick up and transport sediment. However, the relationship is not always straightforward:
    •    High Flow Rates Can Trigger Erosion:
When water is released from a dam at high velocity, it can erode the riverbed and banks downstream. The greater the flow rate, the more energy the water has to dislodge and transport sediment, which can cause channel deepening and bank collapse.
    •    Low Flow Rates Can Increase Erosion Too:
When dams reduce flow rates too much, vegetation may begin to grow along the riverbanks and within the riverbed. If the dam then releases a sudden surge of water, the newly stabilized banks can be rapidly eroded, leading to greater instability.
    •    Erosion Hotspots:
Uneven flow at a dam (e.g., higher discharge at the ends) can concentrate erosive forces in specific areas. This can lead to “erosion hotspots” where sediment is stripped away more rapidly, causing the riverbank to become unstable and more prone to collapse.
    •    Sediment Starvation and Bed Armoring:
Over time, as sediment is trapped behind the dam, the riverbed downstream may become “armored” with larger rocks and gravel. The finer sediment gets washed away, leaving behind only coarse material that is harder to erode. This can create a more stable but less dynamic riverbed.

4. Additional Factors Influencing Erosion and Flow Rate

Several other factors can influence how dams affect erosion and flow:
    •    Dam Design:
The type of dam (e.g., gravity dam, arch dam) and the structure of its spillways and outlets determine how water is released and how energy is dissipated. More even spillway distribution leads to more consistent flow, while concentrated outlets create localized erosion.
    •    Riverbank Composition:
Soft banks composed of sand and silt are more prone to erosion than those made of harder materials like clay or bedrock. The material composition will determine how resistant the banks are to changes in flow rate.
    •    Water Level Fluctuations:
Constant changes in water level caused by dam releases can weaken riverbanks through a process called subsurface erosion (or piping), where water infiltrates the bank and washes away material from within.

5. Testing the Myth

The common assumption that “larger dams always create more consistent and controlled water flow, leading to less erosion” is not always accurate. While larger dams can create more stable flow under certain conditions, the design of the dam, the sediment load, and the characteristics of the riverbanks play a significant role in how erosion occurs. In some cases, smaller dams with uneven flow patterns may cause more localized erosion, especially at the edges where flow is concentrated. This EarthCache will explore these dynamics by comparing the flow patterns and erosion effects at two dams of different sizes along the same river.

Local Implications for the Olentangy River Dams

The two dams on the Olentangy River, located about a mile apart, provide insight into how dam size and design influence flow patterns and erosion. Despite differences in size, both dams regulate the same river and therefore must handle roughly the same volume of water over time. However, the way that water is distributed through each dam creates distinct effects on the riverbanks and flow consistency.

The larger dam, positioned upstream, is wider and likely more capable of distributing flow evenly. However, one side of the larger dam is bordered by a concrete wall, which prevents observable erosion on that side. This reinforces the bank and protects it from the erosive forces of high-velocity discharge. Any erosion from the larger dam would therefore be more visible on the opposite bank, where natural soil and rock are exposed. The even distribution of flow at the larger dam likely reduces concentrated erosive forces, leading to more uniform channel shaping downstream.

In contrast, the smaller dam downstream exhibits higher flow at the edges compared to the middle, which suggests that the spillway or outlet structure concentrates discharge unevenly. This uneven flow increases shear stress on the riverbanks at the edges, causing localized erosion where the water exits with greater force. Over time, this could result in bank undercutting, widening of the channel, and increased meandering of the river.

The presence of the larger dam upstream also influences conditions at the smaller dam. Dams act as sediment traps, and the larger dam likely captures much of the sediment load from the river. As a result, the water reaching the smaller dam is sediment-poor, increasing its erosive power. This sediment deprivation means that the concentrated high-velocity flow at the ends of the smaller dam is more effective at stripping away material from the riverbanks, amplifying erosion at those points.

This relationship highlights how dam size, structure, and positioning interact to shape river behavior. While a larger dam may produce more consistent flow and reduce overall erosion, downstream structures like the smaller dam can still experience intensified erosion due to uneven flow patterns and sediment starvation.

How to Access Both Dams

Since, you must visit both dams to observe and infere, it is only fair that I make your journey between the spots easier. Below I have a trail map with purple as the shortest route, as well as orange, the route I took between stages. This takes you through an incredible park with great hiking trails all around. Parking is on the west side of the highway, and there is tunnel acces to the middle of the lake.

Myth Busting Hypothesis 

After learning about dams above and exploring both dams along the Olentangy River, answer whether you think that, assuming both dams have the same amount of water passing through at once, the larger dam’s more spread-out, uniform flow will result in less erosion.

As this is a team effort, input YES into the checker if you think so, and a NO if not.

You can input your earthcache vote with certitude.