by Daniel Brouse and Sidd Mukherjee
Reservoir Dry Dogs: Climate Change Is Shrinking America’s Water Storage
Climate change is making the world’s reservoirs less reliable, altering both the quantity and quality of stored freshwater. Rising temperatures, erratic precipitation, drought, wildfire, extreme rainfall, and shifting ecosystems are attacking water storage through three interconnected mechanisms:
- Less water entering reservoirs
- Less capacity to store the water that does arrive
- Degrading water quality and creating additional climate feedbacks
The result is a growing climate-water squeeze: the water supply is declining while the infrastructure designed to store that water is simultaneously losing capacity.
The Global Reservoir Problem
More than half of the world’s largest reservoirs and lakes have shrunk significantly since the early 1990s as a result of climate change, extreme drought, human water consumption, and rapid sedimentation.
Research published in journals including Science and Nature Sustainability indicates that nearly two-thirds of existing large reservoirs have experienced substantial water losses.
This is not simply a drought problem. It is increasingly a water-storage problem.
The Colorado River System
Lake Mead and Lake Powell are the two largest reservoirs in the United States, supplied by the Colorado River system.
A multi-decade, climate-change-driven megadrought combined with heavy regional water demand has pushed both reservoirs toward historically low levels. Persistent low storage threatens drinking-water supplies and hydropower generation serving more than 40 million people across the American Southwest.
Other Critical U.S. Reservoirs
| Reservoir | State/System | Primary Role | Primary Climate Threat | Current Impact |
|---|---|---|---|---|
| Choke Canyon | Texas | Municipal drinking water | Intense multi-year drought | ~12% volume; downstream estuary protections suspended |
| Elephant Butte | New Mexico / Rio Grande | Agricultural irrigation | High evaporation & low snowmelt | Frequently near-empty; severe farm-supply deficits |
| Strontia Springs | Colorado | Denver municipal supply | Wildfire erosion & flash floods | Rapid sediment buildup and declining storage capacity |
| San Luis | California / Central Valley | Agricultural & municipal hub | Atmospheric rivers & extreme heat | High runoff pollution, increasing groundwater dependence |
Global Climate-Water Hotspots
The most critically impacted reservoirs and water systems around the world are experiencing intense climate-related strain.
| Reservoir / Lake | Location | Primary Climate Driver | Status / Impact |
|---|---|---|---|
| Lake Mead & Lake Powell | United States | Megadrought & low snowmelt | Drinking-water and energy deficits |
| Balbina Reservoir | Brazil | High heat & decaying biomass | Massive methane emissions and ecological damage |
| Gowd-e-Zareh | Afghanistan | Extreme heat & runoff loss | Widespread desiccation and loss of agricultural water |
| Mar Chiquita | Argentina | Accelerated evaporation & land pressure | Severe shrinkage and ecological threat |
These examples illustrate an increasingly important principle: climate change does not have to eliminate a water source completely to make the water system fail.
Extreme Evaporation
One of the simplest ways climate energy attacks reservoirs is through evaporation.
For an open water surface—including a reservoir or swimming pool—evaporation depends on several interacting variables. A useful framework is the Penman combination equation, which incorporates both the energy available at the surface and aerodynamic drying.
One common form is:
λE = [Δ(Rₙ − G) + ρₐ × cₚ × (eₛ − eₐ) / rₐ] / (Δ + γ)
Where:
- λE = latent heat flux associated with evaporation
- Δ = slope of the saturation vapor-pressure curve
- Rₙ = net radiation
- G = ground or substrate heat flux
- ρₐ = air density
- cₚ = specific heat capacity of air
- eₛ = saturation vapor pressure
- eₐ = actual vapor pressure
- rₐ = aerodynamic resistance
- γ = psychrometric constant
The equation contains two major physical components.
1. Available Energy
The term:
Δ(Rₙ − G)
represents the energy available at the surface to drive evaporation.
Greater available energy generally means greater potential evaporation when sufficient water is available.
In a warming climate, increasing temperatures and changes in surface energy can therefore increase evaporative demand.
2. Aerodynamic Drying
The term:
ρₐ × cₚ × (eₛ − eₐ) / rₐ
represents the aerodynamic component.
The difference:
eₛ − eₐ
is the vapor-pressure deficit (VPD).
A larger VPD means the atmosphere is farther from saturation and therefore has a greater capacity to remove water from a wet surface.
Wind increases turbulent exchange, continually replacing moist air near the water surface with drier air.
The result can be extreme evaporation.
For reservoirs, this means that the same climate energy that produces hotter air can also accelerate the loss of the water stored behind dams.
America’s Reservoirs Are Filling With Sediment
Evaporation and drought are only half of the problem.
Climate change is creating a dangerous feedback loop in America’s water-storage system: drought is reducing available water just as wildfires and increasingly extreme rainfall are accelerating sedimentation that reduces the capacity of reservoirs to store what water remains.
Strontia Springs: A Case Study
Strontia Springs Reservoir near Denver provides a clear example.
After 44 years, nearly 20% of its storage capacity has been consumed by sediment.
Wildfires upstream destroyed vegetation that once stabilized steep slopes. Subsequent rainstorms then washed enormous quantities of soil, sand, and debris into the reservoir.
Denver Water has already spent $18.5 million on dredging, yet removed less than half of the material it hoped to remove.
The reservoir therefore demonstrates an important climate connection:
Wildfire → vegetation loss → erosion → extreme runoff → sedimentation → reduced water-storage capacity.
The National Problem
The problem extends far beyond Denver.
- The United States has roughly 90,000 reservoirs and dams, supplying about 70% of the nation’s water.
- Of critical federal reservoirs surveyed, nearly one-third have lost at least 10% of their capacity to sediment.
- A recent Nature study estimates that the United States has already lost about 8% of total reservoir storage capacity, equivalent to roughly 25.5 million Olympic swimming pools.
- Nearly half of U.S. reservoirs may be more than 25% filled with sediment.
- Because the population has grown, the loss is even more significant on a per-person basis. Estimates indicate that the United States may have already lost up to 44% of its per-capita water-storage capacity since the 1970s.
And smaller reservoirs may actually be in worse condition because they tend to fill with sediment faster and are less comprehensively monitored.
Why Climate Change Makes It Worse
The climate-water system is being squeezed from multiple directions.
Less water coming in
Climate warming → drought → less snow → reduced runoff → less reservoir inflow
Less storage capacity
Climate warming → more wildfire → vegetation loss → erosion → sedimentation → less reservoir capacity
More extreme runoff
Extreme rainfall → larger runoff events → more soil and debris transported into reservoirs
These processes can operate simultaneously.
That is the critical point.
There is less water coming into reservoirs while the reservoirs themselves are losing their ability to store the water that does arrive.
By 2041, wildfires could double sediment accumulation in more than one-third of U.S. watersheds, with nearly all watersheds experiencing at least a 10% increase.
The Economic Consequences
Shrinking reservoir capacity threatens much more than drinking water.
It affects:
- Agriculture and irrigation
- Hydroelectric generation
- Food prices
- Water-utility costs
- Groundwater supplies
- Agricultural insurance
- Capital costs for water infrastructure
When reservoirs run low, farmers increasingly turn to groundwater.
During California’s 2015 drought, for example, farmers spent an additional $600 million pumping groundwater.
This illustrates another feedback:
Surface-water scarcity → groundwater pumping → depleted aquifers → greater long-term water vulnerability.
The costs therefore extend well beyond the reservoir itself.
The Hidden Danger: Reservoirs Don’t Have to Be “Full” of Sediment
One of the most important points is that the percentage of total sedimentation can be misleading.
A reservoir could theoretically retain 80–90% of its original storage capacity while already experiencing a critical operational failure.
Why?
Because water outlets, hydroelectric equipment, and intake structures are generally located toward the bottom of the reservoir.
A relatively modest amount of sediment can therefore interfere with critical infrastructure long before the reservoir appears visually “full” of sediment.
As one geomorphologist has noted, 15% sedimentation may be enough to block an intake.
That is what happened at Colorado’s Paonia Reservoir, where sediment accumulated around the outlet, eventually forcing managers to drain the reservoir and physically remove sediment.
The lesson is important:
Storage capacity is not the same thing as operational capacity.
Why Solving It Is So Difficult
There are several potential responses, but each presents major challenges.
Dredging
Dredging is extremely expensive and technically difficult. Sediment can be abrasive or contaminated, and disposing of millions of tons of material requires permits and suitable locations.
Flushing
Sediment can sometimes be flushed downstream by releasing large quantities of water through the dam.
But that becomes particularly difficult during a drought—the exact time when water is most valuable.
Preventing Erosion
Managing vegetation and reducing wildfire damage upstream can slow sedimentation.
But climate-driven wildfire risk is increasing, making erosion prevention increasingly difficult.
Building New Reservoirs
New reservoirs are expensive, politically difficult, and environmentally controversial.
More importantly, they can become problematic investments if climate change alters the water-supply assumptions on which they were designed.
Building additional storage does little good if the climate is simultaneously reducing the water available to fill it.
The Bigger Picture: A Climate Infrastructure Feedback
America’s reservoir system is an aging piece of infrastructure whose original assumptions are becoming increasingly obsolete.
Most dams were constructed decades ago, with expected useful lives of roughly 50–100 years. Engineers understood that sedimentation would eventually threaten their usefulness.
The problem was largely deferred to future generations.
Now that future has arrived.
The central problem is therefore not simply that reservoirs are filling with mud.
It is that America is simultaneously losing:
1. The water entering its reservoirs
and
2. The physical capacity to store the water that does arrive.
That creates a powerful infrastructure feedback:
Warming → drought and wildfire → reduced inflow + increased erosion → sedimentation → reduced storage → greater water scarcity → greater dependence on increasingly stressed water sources.
And another climate-energy pathway operates alongside it:
Warming → greater available energy + higher evaporative demand → increased evaporation → further water loss.
Together, these processes transform reservoirs from passive infrastructure into active components of the climate-water system.
Reservoir Dry Dogs
The phrase “reservoir dry dogs” captures the emerging problem: water systems can become increasingly thirsty even while the infrastructure built to supply them remains physically present.
A reservoir does not have to disappear to become unreliable.
It can lose water through drought and evaporation.
It can lose storage through sediment.
It can lose operational capacity when sediment blocks intakes and outlets.
It can lose water quality through heat, pollution, and ecological disruption.
And it can become increasingly expensive to maintain precisely when communities have fewer water resources available to pay for that maintenance.
The climate signal is therefore not simply less water.
It is less reliable water storage.
The deeper climate-energy story is that warming is attacking the water system from both sides:
Less water coming in.
Less room to keep it.
More energy driving it out.
That is how climate energy hits you—even when the water is sitting behind a dam.
