Adapting to Change: Recommendations for Improving U.S. Army Corps of Engineers Reservoir Management

June 20, 2018 | Water in the West | Insights

Letty Belin

How can the United States Army Corps of Engineers improve its reservoir management to adapt to climate change and meet more water needs while avoiding any increased flood risks?

The U.S. Army Corps of Engineers (Army Corps, Corps) operates hundreds of dams and reservoirs throughout the U.S.  An abundance of data confirms that the standard way the Corps currently operates many of its reservoirs is outdated and not well suited to respond to current and future impacts from climate change.  Continuing to strictly follow the Corps’ outdated Water Control Plans and Manuals (WCMs) will mean that flood risks will increase, ability to meet water demands will decrease and downstream ecosystems will deteriorate, unless changes are made. 

The Corps has a surprising amount of discretion to alter its reservoir management, but will generally only do so when three conditions are met:  First, there must be strong support for—or at minimum no objection to— change from all key stakeholders; second, the changes must not be so large as to require Congressional authorization; and third, those stakeholders must supply most or all of the funding necessary to investigate and initiate the requested changes.  If any of these factors is absent, getting the Corps to update and revise its reservoir operations is likely to be a long, and potentially unsuccessful, uphill battle.  In contrast, the Corps has demonstrated that it can be a very effective partner when presented with opportunities to work collaboratively with other agencies and stakeholders to address the challenges posed by climate change and increased water demands, without having to identify substantial amounts of funding for a new undertaking.


Why Army Corps Reservoir Management Matters

Estimates of the number of dams and reservoirs owned and operated by the Corps in the U.S. range from about 500 to 700. (The variability in this estimate may be partially explained by inclusion/exclusion of combined locks and dams.)  According to the Corps, those reservoirs:

  • supply about 25% of hydropower in U.S.;
  • provide municipal and industrial water to 10 million people in over 100 cities;
  • and provide irrigation water in 12 states.

In addition, the Corps manages flood control space in about 130 other reservoirs, including 20 of the largest reservoirs in California (Oroville, Shasta, New Melones, and New Don Pedro, among others).  Reservoirs managed by the Corps account for 37% of all the water storage (over 790 million acre-feet of water) provided by dams in the U.S.


An Outdated Approach

The Corps operates these reservoirs according to WCMs that are developed based on precipitation and water data from the decades preceding construction of each dam.  These WCMs include reservoir “rule curves” and guidelines (see figure 1) defining seasonal limits of water storage at each reservoir, and how water is to be allocated between flood control storage and conservation storage (and storage for any other authorized purpose), thereby guiding storage and releases at each reservoir.  WCMs are required to include Drought Contingency Plans (DCPs) to allow for modifications to operations to increase Corps capability of responding to a drought.  The Corps is in the process of updating DCPs to address appropriate operational modifications to increase each project’s capacity to respond to the effects of climate change.

Figure 1: Rule curves commonly used in reservoir operations. Curves in the top panel are used in flood control and multipurpose reservoirs. Water levels are low during wet months and raised as precipitation subsides to accommodate various demands for the water. Conversely, water levels could be held stable (dashed line) if flood risk is low. The middle panel shows water level increases that are stored for a short time and slowly allowed to decrease to normal levels. This type of curve is commonly used in flood-control reservoirs and in maintaining conservation flows downstream. The bottom panel shows water recharging the reservoir during winter and spring and depleted again in summer with consumptive uses such as agriculture. Source: Mower and Miranda, 2013.



Two-thirds of the dams managed by the Corps were built more than 50 years ago and only a handful of Corps-funded dams have been built since 1990.  The studies and data that provide the basis for the Corps's WCMs for those dams comes from the first half of the 20th century and do not take into account the changing climate.

Multiple studies have found that expected impacts from climate change in the western U.S. will pose significant challenges to reservoir operations by changing snow-rain fractions, snowpack volumes, runoff timing and peak flows, as well as by greater climate variability overall.  Predictions are that flood risk will increase due to more intense precipitation events and more precipitation falling as rain and not snow.   The anticipated reductions in snowpack storage and other climate-induced changes will introduce new challenges to meeting both water supply and flood control goals, as well as other goals such as fish and wildlife conservation and recreation.  These anticipated conditions will differ (and already do differ) significantly from the historic conditions on which the Corps’ WCMs and DCPs are based. 

Not only do old Army Corps reservoir rule curves not account for climate change impacts, but they also fail to take advantage of modern technical tools that may allow for more flexible reservoir management.  These include greatly improved weather forecasting capabilities that can, for example, give advance notice of weather phenomena such as atmospheric rivers, as well as enhanced data on other indicators relevant to reservoir storage such as amount of snow storage and water content in the upstream basin.

All of the challenges facing Army Corps in reservoir operations will be enhanced by the ever-increasing sedimentation of the reservoirs and the resulting shrinkage of storage space.  In addition, sedimentation reduces reservoir discharge capacity and flood attenuation capabilities (needed for flood management), thereby increasing flood risk.  For example, a 2016 study found that the 24 federally-operated reservoirs in Kansas, with an average age of 52 years, had lost 17% of their original capacity due to sedimentation.  Impacts of climate change (e.g., increased heavy precipitation events, more fires) are expected to increase sedimentation rates in at least some western locales.


From Rule Curves to WCM Review: The Need for Adaptability in Army Corps Reservoir Management

A number of technical studies have concluded that the older “static” rule curves governing operations at many reservoirs perform less well than newer “dynamic” rule curves that take other factors such as snowpack, basin wetness, and weather forecasts, etc., into account in the face of climate change.  The studies have found that Corps project operations following dynamic rule curves that take climate change into account are better able to achieve multiple project goals without increasing flood risk.

The Corps is well aware of the benefits provided by a dynamic and adaptive approach to reservoir management and tries to ensure this approach is followed.  For instance, Corps Engineer regulations call for revising WCMs as needed on an ongoing basis and for updating WCMs at least every ten years, to ensure that they do not get outdated by changing conditions.  Records kept by Corps officials generally do not document regular formal updates of WCMs, however.  Instead, Corps officials report that they informally review WCMs as part of their daily project operations and do revisions as needed.  According to a 1990 report, the WCMs for many reservoirs have not been updated since construction, and the GAO found in 2016 that most WCMs have not been formally updated in more than 10 years.

The fact is that the Corps lacks the resources and staff to review and update WCMs every 10 years.  As a general matter, the more extensive or complex the revision of a WCM, the more technical and environmental reviews must be done for it, which translates in to higher costs and more time.  Costs of WCM revisions range from tens of thousands of dollars and short turn-around times to millions of dollars and delays of multiple years.  Revising the Master WCM for Missouri River dam operations, possibly the most complex WCM in the country affecting many states and stakeholders, took decades and cost over $10 million.  DCPs are easier to amend than WCMs but that process also may require National Environmental Policy Act (NEPA) documentation if impacts might occur that were not the subject of previous NEPA analysis.

As one Corps official states, “funding is scarce” for updates to the “many manuals [that] are decades old.”  While the Corps has identified WCMs needing revision, funding has gone instead to other more immediate priorities such as levee and equipment repairs and sediment removal. 


The Authority to Adapt

As long as the Corps avoids “major operational changes” to reservoir operations (defined as reallocation of either 50,000 acre-feet of storage space or 15% of total storage space), it does not need to obtain Congressional approval for exercising flexibility in reservoir operations in response to drought and climate change impacts. While the Corps has the authority to revise rule curves set forth in WCMs, as noted above, the real question is “what is the Corps most likely to be able to do in a timely fashion?”

First, under drought conditions, if optimal reservoir operations do not accord with the WCM rule curves under drought circumstances, DCPs may allow for suspension or relaxation of rule curves or otherwise give the Corps flexibility to alter reservoir operations temporarily.  This is the most efficient strategy, as it requires neither a deviation nor additional environmental review. 

Second, WCMs provide a process for allowing the Corps to deviate temporarily from WCM rule curves.  Deviations may be used to alleviate critical situations or to increase project benefits without significantly affecting the project’s ability to achieve its authorized purposes.  There are three levels of permissible deviations: “emergency” (to mitigate an immediate threat to public health and safety, property, the project or the environment); “planned” and “unplanned” (neither an emergency nor planned).  Deviations are approved at the Division level unless delegated to the District level.  Deviations may be requested for three consecutive years or three times in five years before higher level review is required and WCM revisions may be necessary.

The damaged Oroville Dam spillway on February 26, 2017 after heavy precipitation events cause the dam to overflow.



If a deviation involves changes beyond the scope of what was analyzed in previous environmental review documents, further NEPA review is required.  As a practical matter, a deviation that does not risk significant adverse environmental effects, that is consistent with authorized purposes, and that does not give rise to any major objections, can occur in a relatively short time frame.  Ultimately, if deviations are successful and it is anticipated that conditions in the future will continue to warrant them, their provisions can be incorporated permanently into the DCP or elsewhere in the WCM. 

One Corps official offered the following as advice on how to allow the Corps to quickly alter its reservoir operations in response to climate change:

The quickest way to change a reservoir’s operations would be a series of adaptive measures requiring minor regulation changes that have no negative impact on the authorized purposes of the reservoir and no significant adverse impacts per NEPA.

As the Corps acknowledges in a 2015 document addressing drought contingency planning, “[t]he combination of water control manuals, DCPs, and deviations provide a great deal of flexibility to respond to short-term and long-term needs based on best available information and science.”


Including Stakeholders in Army Corps Reservoir Management Revisions

In recent years there have been several collaborative efforts among the Corps, water users and other stakeholders to test and implement revisions to reservoir operations to better meet project goals without impairing flood protection.  Two such initiatives to test “Forecast-Informed Reservoir Operations” (FIRO) are underway in California at Lake Mendocino and Prado Dam.  The FIRO initiative at Lake Mendocino, which began in 2014, predicts a 15-35% increase in water supply as a result of the revised operational rules (implemented through short-term deviations) that, in particular, take into account “atmospheric rivers” and related weather phenomena.  The agencies collaborating on this effort include the Bureau of Reclamation, the National Oceanographic and Atmospheric Administration, the U.S. Geological Survey, the California Department of Water Resources, and UC San Diego’s Scripps Institution of Oceanography, in addition to the Corps and Sonoma County Water Agency.

Major General William T. Grisoli, Deputy Commanding General for Civil and Emergency Operations for USACE meets with Mike Finnegan, area manager for the Bureau of Reclamation's Folsom Dam office.

In Southern California the Orange County Water District has been collaborating with the Corps in requesting a 5-year operational deviation that began this water year to allow increased groundwater storage behind Prado Dam, which is expected to increase groundwater storage by an average of 6000 acre feet per year.  In both cases, while the Corps is providing the necessary staff work, other entities are providing funding to support the initiative.

Another innovative Corps initiative has been a long-standing partnership with The Nature Conservancy called the “Sustainable Rivers Project.”  This undertaking, which also involved funding and support from other stakeholders, has led to incorporating “environmental flows” into dam operations on the Green River in Kentucky, the Bill Williams River in Arizona, and the Savannah River in Georgia and South Carolina.  To date, the minimal amount of Corps funds available for this initiative have limited its scope to only a handful of rivers.


Bringing It All Together: Key Factors for Success in Achieving Flexible Management of Army Corps Reservoirs

The examples above demonstrate that significant beneficial revisions that can be made to Corps reservoir operations when there is strong, committed stakeholder support, the revisions sought are not so large as to require Congressional action, some initial funding has been provided by stakeholders and there is no significant opposition to the initiative.  Certainly, these circumstances are not often present in many places where updates to WCMs are sorely needed and climate change is already having detrimental effects.  But the examples cited above demonstrate that real benefits can be achieved when parties directly affected by Corps reservoir operations collaborate with the Corps to come up with ideas on how to improve reservoir operations in a way that benefits all parties concerned.  Even as we press Congress and the Corps to adopt a wholesale approach to this problem, let us also focus on those places where, in the meantime, we can make meaningful improvements that benefit large numbers of people as well as river ecosystems.