Brantley Dam: A New Look for the Project
The Carlsbad District's water storage problems remained a significant issue into the 1960s. Silting at McMillan Reservoir continued to inhibit the facility's storage capacity, and concerns over McMillan's structural safety persisted. The safety issue generated increasing Federal attention as the years went by. In 1964, the Bureau of Reclamation prepared a "Safety Evaluation Study" of McMillan and Avalon, which concluded that "a potential flood would exceed existing spillway capacity at McMillan Dam and cause the dam to be overtopped, which would cause the failure of both dams."  This dire forecast finally roused the Federal government to action action which would dramatically alter the infrastructure of the Carlsbad Project.
Extensive Bureau of Reclamation research on possible solutions to the Carlsbad Project's shortcomings culminated in a massive 1967 report proposing the replacement of McMillan Dam and Reservoir with a new structure. The new dam would be located between Avalon and McMillan, creating a large new reservoir that would completely inundate the McMillan site. The proposed facility was designated Brantley, after an early officer of the Carlsbad Irrigation District. Although Brantley's site was upstream from the long-proposed Reservoir No. 3, the proposal was, in many ways, a final implementation of the nearly century-old idea.
The 1967 report was intended, in part, to serve as a basis for securing Congressional authorization of the Brantley project. This approval came on October 20, 1972 in the form of Public Law 92-514, which authorized the Department of the Interior's construction and future operation of Brantley Dam and Reservoir. The project's stated goals included "the purposes of irrigation, flood control, fish and wildlife and recreation, and for the elimination of the hazards of failure of McMillan and Avalon Dams."  The authorization was contingent on the Bureau of Reclamation's retention of Alamogordo Dam and Reservoir, and on the bureau's establishment of procedures that would "preclude any detrimental effect on water rights in the Pecos River."  A total of $45,605,000 in Federal funding was initially authorized for the project. 
Planning work for Brantley Dam continued throughout the 1970s. Changes made during this period included relocating the dam's axis to help satisfy geological concerns and increasing the size of the reservoir's minimum pool to improve wildlife habitat. A further change was the decision to breach McMillan Dam after Brantley's completion. (Original plans had called for the relocated Atchison, Topeka & Santa Fe Railway to cross Brantley Reservoir atop the abandoned McMillan Dam.) 
The financial responsibility for Brantley's construction was divided between the Federal government and the Carlsbad Irrigation District. The irrigation district would shoulder only those project costs associated with providing additional irrigation water (estimated at $1,066,000 in 1975), while the bulk of Brantley's costs, associated with flood control and dam safety, were assumed by the Federal government. Total estimated project costs climbed steadily as the plans for Brantley were refined. By 1982 the estimated costs of the project had risen to $218,300,000, and they rose still higher as the decade progressed. Interestingly, the 1982 project budget reduced the amount chargeable to the irrigation district to only $157,000. 
The final plans for Brantley Dam specified a structure combining a concrete gravity center section flanked by rolled earth-fill wings. The concrete section is approximately 143.5 feet high and 760 feet long. The dam's earth-fill wings reach a maximum height of 118.5 feet and have a crest width of 30 feet. The wings extend both east and west of the concrete core, bringing the total dam length to some four miles. The concrete section features a central overflow spillway controlled by 6 radial gates, providing a maximum discharge capacity of 352,000 cubic feet per second. The spillway supplements the dam's regular outlet works, which consists of two 4-foot by 4-foot conduits with a capacity of 1,230 cubic feet per second. 
Preparatory land acquisition, site clearing, and archeological mitigation work was underway at the Brantley site by the early 1980s. This phase of work also included relocating highways and utility lines, and constructing a $15,000,000 realignment of the Santa Fe Railway. Monterrey Construction Company, the Bureau of Reclamation's prime contractor for the project, began actual construction of the dam in 1984. Concrete work was completed by autumn 1987, and soon after the Pecos River was diverted through the new Brantley floodgates for the first time. That winter, still another channel was dredged through the McMillan silt to ease the water flow into Brantley. 
The completion of Brantley Dam heralded a significant new era in the history of the Carlsbad Project. Brantley's construction finally allowed long-standing local concerns about dam safety to be laid to rest, and the Project's inadequate water storage capacity was simultaneously resolved. Unlike McMillan, Brantley was designed in anticipation of future siltation, and the dam is high enough to allow for gradual raising of the reservoir pool's elevation to compensate for silting as it occurs. In a demonstration of the concept of multiple-use, plans for Brantley Reservoir also included active consideration of the recreational opportunities and waterfowl habitat the new lake would provide. 
The use of Brantley for project water storage allowed the final abandonment of McMillan Dam and Reservoir, and in 1990 plans were underway to breach the historic structure. McMillan's abandonment leaves the Carlsbad Project with an eclectic mixture of physical facilities: nineteenth-century canals mixed with twentieth-century lining and headgates. the striking 1903 Pecos River flume, the 1906 Avalon Dam with its later improvements, the 1937 dam at Alamogordo, and modern-day Brantley. In design and function, each of these structures represents the era in which it was built, and the form of the Carlsbad Project at a unique point in its past. Together they display a significant technological cross-section of the history of western American reclamation.
The slow, painful evolution of the Carlsbad Project under Bureau of Reclamation tutelage mirrors, in many ways, the advancing maturation of American reclamation technology and philosophy. Reclamation Service efforts to reconstruct Avalon in 1906 and 1907, combined with its later improvements to the dam, are today highly representative of the increasing sophistication of early 20th-century reclamation engineering. Avalon and McMillan Dams remain especially significant for their rockfill, composite design, and Avalon's use of an impervious corewall is also noteworthy.
By the beginning of the Reclamation Service era, dam engineers recognized the advantages of placing a corewall in earthfill and rockfill dams. The impervious corewall, usually of steel sheet piling or concrete, was carried from bedrock into the fill, thus restricting the flow of water at the interface between the earthfill and its foundation. Although at least one Reclamation Service engineer recommended that all corewalls be carried to the crest of the dam to protect against burrowing animals, in some early Reclamation Service projects the corewall was carried only a few feet upward into the fill.
Of the 37 major Reclamation Service dams and diversion dams described in the volume Irrigation Works Constructed by the United States Government, Avalon Dam is one of only three rockfill dams with an impervious earthfill upstream facing, and the only rockfill dam with a corewall extending from bedrock to the dam's crest. The other two dams of similar construction were the Minidoka Dam on the Minidoka Project in Idaho and the Clear Lake Dam on the Klamath Project in California and Oregon. Both of these dams had concrete corewalls only at the base of the upstream toe of the rockfill. Avalon Dam was comparable in size to Minidoka and Clear Lake, although its reservoir was far smaller than that of the others. Similarly, Minidoka and Clear Lake were components of far larger Reclamation Service projects, each helping supply water to hundreds of thousands of acres. 
As the 20th century progressed, rockfill dams continued to be popular in certain applications and, while some composite dams were built, most had concrete membranes on the upstream face. Among rockfill dams with an earthfill facing, Avalon and McMillan were among the largest 19th-century dams in terms of length and height, but there were other 19th-century dams with impermeable cores or with concrete or timber upstream faces which greatly exceeded the Pecos River dams in height. Later in the 20th century, rockfill dams with an earthfill facing would reach heights of almost 200 feet. Nevertheless, Avalon and McMillan are significant because they were among the first important rockfill dams with an earthfill facing in the United States. 
Less-prominent features of the Carlsbad Project's physical plant also represent the early technological evolution of the Bureau of Reclamation. The most noteworthy of these are Avalon Dam's 1911 cylinder gates, an early implementation of a design form which saw later use in the intake towers of Hoover Dam. The Avalon project apparently marked the earliest use of the cylinder gate design in a major dam spillway.
In addition to the Carlsbad Project's physical landmarks, its operational history also typifies the evolution of public-sponsored reclamation in the American West. The project's long standing attempt to maintain fiscal responsibility without unduly burdening project farmers replicates the primary concern of most Reclamation Service projects, and Carlsbad's relative success in achieving such a financial balance made the project a source of Federal pride. Carlsbad is also nationally representative in its long-discussed, but slowly implemented, transition from Federal to local control.
Still, a number of factors make the Carlsbad Project historically unique. While the Reclamation Service imposed many of its contemporary designs on the project, these features were added to an irrigation system of private origin and largely 19th-century construction. This resulted in a physical plant displaying true dichotomies of design and philosophy, exhibiting both private and public design philosophies, as well as both 19th- and 20th-century engineering techniques. While the Pecos Valley was not alone in possessing a privately-constructed irrigation system that was later acquired by the Federal government, the relatively massive size of the private system was unusual among such projects. Although the Federal government's construction and rehabilitation investments at Carlsbad were substantial, during the Reclamation Service's first years the Project largely remained a private, 19th-century system imbued with the trappings of Federal funding and control. Although this characterization gradually faded with each government improvement project, the dichotomy remains sharply visible today.
10. Davis, Irrigation Works Constructed by the United States Government; Department of the Interior, Bureau of Reclamation, Reclamation Project Data (Washington: Government Printing Office, ); Hall, "Reinforced Concrete Diaphragms for Earth Dams," 145-146.
Last Updated: 01-Feb-2008