This report presents the risks and actions necessary to reduce risks associated with dam safety issues identified at Horsetooth Dam, Soldier Canyon Dam, Dixon Canyon Dam, and Spring Canyon Dam, collectively referred to as the Horsetooth Reservoir Dams. This report presents various alternatives to reduce the risk to the public; identifies associated potential economic, environmental, and social impacts of each of the alternatives; and identifies a preferred alternative. A Final Environmental Assessment (EA) which supports a Finding of No Significant Impact (FONSI) and fulfills the requirements of the National Environmental Policy Act (NEPA), accompanies this report.
The Horsetooth Reservoir Dams are features of the Colorado-Big Thompson Project. Construction funds were first provided in the Interior Department Appropriation Act of August 9, 1937 (50 Stat. 595). The Secretary of the Interior's finding of feasibility was approved by the President on December 21, 1937.
The modifications proposed for the Horsetooth Reservoir Dams are authorized by the Reclamation Safety of Dams Act of 1978 (Public Law 95-578), as amended by the Reclamation Safety of Dams Act Amendments of 1984 (Public Law 98-404)(The Act) and future authorizing legislation which would provide sufficient appropriation ceiling to complete the risk reduction actions at all four dams. The Act authorizes the Secretary of the Interior to construct new or modified features at existing Reclamation dams for safety of dams purposes. The proposed modifications to Horsetooth Reservoir Dams effectively reduce the risk to the public and preserve the present project benefits. Project benefits are not enhanced with the proposed corrective actions.
Reclamation's appropriation ceiling is not sufficient to allow funds to be appropriated to complete all risk reduction measures at the four Horsetooth Reservoir Dams. Legislation needs to be enacted to provide sufficient appropriation ceiling to complete all of the proposed risk reduction actions.
The Colorado-Big Thompson Project is one of the largest and most complex natural resource developments undertaken by the Bureau of Reclamation. It consists of over 100 structures integrated into a trans-mountain water diversion system through which multiple benefits are provided.
The project spreads over approximately 250 miles in the State of Colorado. The location and extent of the project is depicted on the project map at the beginning of this report. It stores, regulates, and diverts water from the Colorado River on the western slope of the Continental Divide to the eastern slope of the Rocky Mountains. It provides supplemental water for irrigation of about 720,000 acres of land, municipal and industrial use, hydroelectric power, and water-oriented recreation opportunities.
Major features of the project include dams, reservoirs, powerplants, pumping plants, pipelines, tunnels, and other associated structures. A more complete description of the project features is included in the Appendix .
Horsetooth Dam, Soldier Canyon Dam, Dixon Canyon Dam, and Spring Canyon Dam impound Horsetooth Reservoir and are features of the Colorado-Big Thompson Project. The Horsetooth Reservoir Dams were constructed between 1946 and 1949. The reservoir is situated in a narrow valley west of a "hogback" ridge and extends in a north-south direction immediately to the west of Fort Collins, Colorado. The Horsetooth Reservoir Dams are Reclamation facilities operated and maintained by the Northern Colorado Water Conservancy District.
Horsetooth Reservoir impounds 156,735 acre-feet at normal reservoir water surface elevation 5430. Water is supplied to Horsetooth Reservoir by the Charles Hansen Feeder Canal which has a maximum capacity of 550 ft3/s and from precipitation on a relatively small drainage area of 17.5 square miles. Releases from the reservoir for irrigation, municipal, and industrial uses are controlled by outlet works structures at Horsetooth and Soldier Canyon Dams. Because of the small drainage area, no spillway was constructed for Horsetooth Reservoir. The crest elevation of all four dams and the dike was raised approximately 3 feet during the period from April 1988 to September 1989 to elevation 5443.0 to contain the probable maximum flood. This was accomplished by adding embankment material to the crest of each embankment with varying steepened slopes on the upstream and downstream faces near the crest of the dams. All four dams are essentially homogeneous embankments except for thin downstream rock fill and gravel fill zones and a riprap layer on the upstream face.
Horsetooth Dam has a structural height of 158 feet, a crest width of 36.5 feet, and a crest length of 1,860 feet. The upstream slope is 3:1 (H:V) from an intersection with the steepened 1.5:1 slope down to the upstream toe. The upstream face of the dam is protected by zone 4 rock fill down to elevation 5380 and by 3 feet of riprap to the toe. The downstream slope of the dam is 2.5:1 from the steepened section to elevation 5350, 6:1 to elevation 5325, and then 1.5:1 to the downstream toe. The downstream face is protected by zone 4 rockfill in the steepened section and by zone 3 sand, gravel, and cobbles to the downstream toe.
Satanka Dike is adjacent to the left abutment of Horsetooth Dam. It has a structural height of 33 feet, a crest width of 26.5 feet, and a crest length of 415 feet. The upstream slope is 3:1 below an intersection with the steepened slope from the crest. The upstream slope is protected by zone 4 rockfill in the steepened section and 3 feet of riprap on the 3:1 slope. The downstream face of the dike has a 2:1 slope from the crest to the downstream toe and is protected by zone 4 rockfill. The embankments of Horsetooth Dam and Satanka Dike contain a total of approximately 1,894,000 yd3 of material.
Soldier Canyon Dam is located in the northernmost V-shaped notch in the "hogback." The dam has a structural height of 229 feet, a crest width of 36.5 feet, and a crest length of 1,445 feet. The upstream slope is 2.5:1 below the intersection with the steepened slope down to a 20-foot berm at elevation 5300, then 5:1 to the upstream toe. The upstream face is protected by zone 4 rockfill from the crest down to elevation 5330 and riprap down to the berm at elevation 5300. The downstream face has a slope of 2.5:1 from the steepened section down to elevation 5250, then 6:1 to the downstream toe. The downstream face is protected by zone 4 rockfill in the steepened portion and by zone 3 sand, gravel, and cobbles to the toe. Soldier Canyon Dam contains approximately 3,224,000 yd3 of material.
Dixon Canyon Dam is sited in the center V-shaped notch in the "hogback" and has a structural height of 243 feet, a crest width of 36.5 feet, and a crest length of 1,270 feet. The upstream slope is 2.5:1 below the intersection with the steepened slope down to a 20-foot berm at elevation 5300, then 5:1 to the upstream toe. The upstream face is formed of and protected by zone 4 rockfill from the crest to the 20-foot berm. The downstream face has a slope of 2.5:1 from the intersection with the steepened slope down to elevation 5250, then 6:1 to the downstream toe. The downstream face is formed of and protected by zone 4 rockfill from the crest to the toe. Dixon Canyon Dam contains approximately 2,981,000 yd3 of material.
Spring Canyon Dam is sited in the southernmost V-shaped notch in the "hogback" and has a structural height of 223 feet, a crest width of 36.5 feet, and a crest length of 1,140 feet. The slopes and zoning of this dam are very similar to Dixon Canyon Dam.
Two outlet works structures were constructed for Horsetooth Reservoir. One structure is at Horsetooth Dam and the other is at Soldier Canyon Dam. The outlet works at Horsetooth Dam is a conduit located just to the right of the middle of the dam. The structure consists of an inlet structure with trashracks, an 8-½ foot-diameter concrete conduit, a gate chamber containing two 5- by 5-foot high-pressure emergency slide gates, a double-barrel 10-foot-diameter horseshoe downstream conduit with two 72-inch-diameter steel pipes, a valve house containing two 72-inch hollow-jet valves for controlling flows, and a stilling basin which discharges into the Charles Hansen Supply Canal. The Horsetooth outlet works has a discharge capacity of 2,500 ft3/s at reservoir water surface elevation 5430, even though the capacity of the Charles Hansen Supply Canal downstream is only 1,500 ft3/s.
The Soldier Canyon outlet works is located in a tunnel through the right abutment. The structure consists of an inlet structure with trashracks, a 5-foot-diameter circular concrete tunnel, and a gate chamber containing a 54-inch butterfly valve. A shaft house on the dam crest and adit allow access to the gate chamber. A 54-inch-diameter steel pipe encased in a concrete tunnel extends downstream to the control house. The control house contains a 48-inch butterfly valve for emergency closure of the pipeline that supplies the Colorado State University Hydraulics Laboratory, a 54-inch butterfly valve for emergency closure of the pipeline supplying the city of Fort Collins, and a 16-inch emergency butterfly valve and a 16-inch sleeve valve in a stilling well which regulates flow into the Dixon Feeder Canal.
The Horsetooth Reservoir Dams have been formally classified as a high hazard facility. The dams impound 156,735 acre-feet at normal pool and are situated immediately above the city of Fort Collins, with a population of approximately 100,000 residents, and Colorado State University, with an enrollment of approximately 30,000 students. A failure flood from the various Horsetooth Reservoir Dams would affect numerous communities, farmland, developed areas, many roads, and Interstate I-25 along the Cache la Poudre River and South Platte River. Some of the communities that could be affected by a failure flood in addition to Fort Collins are Timnath, Windsor, and Greeley, Colorado.
Plan and section drawings of the dams as originally constructed are included in the Appendix of this report.
In 1870, before statehood was achieved by the Colorado Territory, the Union Colony of 2,000 people was established at Greeley. This marked the inception of cooperative irrigation in the South Platte River Valley and the beginning of an era in which irrigation became important in the economic development of northeastern Colorado.
The Union Colony started with construction of ditches to supply direct flow from the river to 12,000 acres. The venture was so successful that by 1900 the streams were over appropriated and attention was given to developing of plains reservoirs to store the spring floods. By 1910, most of the better reservoir sites were used and few possibilities were apparent, except costly transmountain diversion.
During these years, the increasing demand for agricultural products for a growing population and the tendency to prepare as large an irrigation system as possible to spread the cost of the works, resulted in over-expansion, especially in years of high and adequate runoff. Subnormal or even normal runoff years were critical for much of the area so developed. Water shortages continually plagued the irrigators.
The idea of transmountain water diversions had been in existence since 1889, when the Colorado legislature appropriated money to investigate such a proposal. Progressive steps in legislation finally led, in 1922, to the signing of the Colorado River Compact, which apportioned the Colorado River water between the upper and lower basin States. Later, the Boulder Canyon Act provided funds for determining the amount of lands that were or could be irrigated in the Colorado River Basin. A plan was developed whereby Colorado River water could be diverted into watersheds in northeastern Colorado where there was a surplus of irrigable lands and a shortage of water. The upper basin States successfully developed a compact in 1948 prorating the upper basin's share based on the 1922 compact.
Engineering investigations of the Colorado-Big Thompson Project began in 1933, when a preliminary survey to determine the feasibility of a project was undertaken. A favorable report was presented in 1934. In January 1935, the Bureau of Reclamation was allotted funds by the Public Works Administration to make a new study.
Project construction was contingent upon the formation of a conservancy district to contract with the United States Government. Accordingly, the Colorado Water Conservancy Law was passed by the Colorado legislature in 1937. The law contains several unique features. One provides that a conservancy district may be organized by any district court upon petition of a stipulated number of property owners; another recognizes that all who benefit as a result of project development should contribute to its cost and operation in proportion to those benefits.
The Northern Colorado Water Conservancy District was organized in 1937 with boundaries which include large areas of Larimer, Boulder, and Weld Counties, and portions of Morgan, Washington, Logan, and Sedgwick Counties.
Construction of the project began at Green Mountain Dam during November 1938. The first power was generated at the Green Mountain Powerplant in May 1943; all construction of the dam and powerplant was completed in October 1943. Construction of Granby Dam started in 1941, and of Alva B. Adams Tunnel in the summer of 1940. Work was curtailed during World War II, but not entirely stopped. At the end of the war, the tempo of construction was accelerated. By 1956, all major features including the Horsetooth facilities were essentially completed except the Big Thompson Powerplant, which was completed in 1959.
The Colorado-Big Thompson Project helps stabilize the agricultural and industrial economy of northeastern Colorado. It is particularly effective each year during late summer months of the irrigation season, and has a tremendous impact throughout the season in drought years.
Principal crops irrigated with project water include barley, corn (grain and silage), oats, sorghum (grain), wheat, alfalfa, other hay, dry beans, sugar beets, cabbage, carrots, sweet corn, cucumbers, onions, potatoes, and other miscellaneous field crops and vegetables. Other usages include irrigated pastures.
Municipal supplies have been an important aspect in the distribution of project water. Originally, nine communities had allotments totaling 44,950 acre-feet. Eleven communities now receive full or supplemental supplies. Each year, as urban population increases, irrigation allotments are transferred to domestic purposes. The dependable availability of water continues to attract a variety of industries.
The economic analyses in this report utilize the Federal Fiscal Year 2000 discount rate of 6.625 percent and a remaining dam life of 50 years to convert annual monetary values to capitalized present values. Analyses conform to the Economic and Environmental Principles and Guidelines for Water and Related Land Resources Implementation Studies of March 1983 (P&G's).
Horsetooth Reservoir is an integral part of the Colorado-Big Thompson (CBT) Project, which collects and diverts water from the upper reaches of the Colorado River west of the Continental Divide, and conveys it to the East Slope for storage and use. All water diverted from the West Slope to the East Slope must be put to beneficial use for irrigation or municipal and industrial (M&I) purposes, or it cannot be diverted. Horsetooth Reservoir is the primary East Slope storage facility for the provision of irrigation and M&I water to the northern portion of the CBT project. The CBT project also produces hydroelectric power at one power plant on the West Slope and at five hydropower plants on the East Slope. Although there is no power plant at Horsetooth Reservoir, all diversions of irrigation and M&I water from the West Slope to the East Slope are also routed through the CBT Project power plants. Thus, any changes in diversions for M&I and irrigation purposes also affect CBT Project power generation. The reservoir also has storage capacity for potentially damaging flood waters.
There has been a continuing transition in the use of CBT water from irrigation to M&I purposes. In 1970, water use was approximately 90 percent irrigation and 10 percent M&I water. Current use is approximately 70 percent irrigation and 30 percent M&I water. Due to the expected continued rapid growth of urban areas within the CBT Project area, a trend line was developed based on historical data which projects a future conversion rate from irrigation to M&I use of about 0.75 percent of total water supply annually. This trend in the transition of project irrigation water to M&I use was reflected in the estimation of future CBT water benefits attributable to Horsetooth Reservoir.
In order to determine total economic costs of the various alternatives to address dam safety issues at the Horsetooth Reservoir Dams, it was necessary to address impacts the reservoir has on the value of water the project provides for purposes of irrigation, M&I water, hydropower, recreation, and flood control. Flood control benefits were not quantified for this analysis due to a lack of relevant data. Because of the integrated nature of the CBT Project, elimination of Horsetooth Reservoir would result in the loss of only a portion of the CBT Project benefits. To determine the economic impacts of Horsetooth Reservoir, CBT operations were simulated based upon historic records. The computer simulation provided an idealized representation of potential reoperation of the CBT without Horsetooth Reservoir. Because the reoperation is an idealized simulation, the documented impacts of CBT operations without Horsetooth Reservoir are likely understated. Using the analysis described below, the combined capitalized present worth of CBT Project benefits provided by or controlled by Horsetooth Reservoir is about $696 million. The following narrative addresses each of the benefit areas.
a. Irrigation Benefits- The CBT Project as a whole provides a supplementary water supply for about 720,000 acres of land. Irrigation water deliveries from Horsetooth Reservoir, which currently average about 87,700 acre-feet annually, serve about 53,000 acres in the northern portion of the CBT Project. However, if Horsetooth Reservoir was eliminated, i.e., if any one of the Horsetooth Reservoir Dams was breached, the CBT would be reoperated to continue to provide about 67,210 acre-feet of water to these lands. Therefore, in effect, current net annual irrigation water provided by the reservoir is about 20,490 acre-feet. As addressed above, future use of irrigation water was gradually reduced to reflect the continued transition of water use from irrigation to M&I use. The projected increased use for M&I purposes resulted in a reduction in irrigation usage to about 9,430 acre-feet at the end of the 50-year projection period.
Irrigation benefits are based on the net increase in the value of project production due to the application of project irrigation water. Estimation of economic benefits provided by this water was accomplished using a farm budget methodology, which modeled the operations of a representative farm with and without irrigation water. The representative irrigated farm consists of about 550 acres of irrigated crop land. Principal crops include corn, alfalfa, corn silage, sugar beets, dry beans, and wheat, as based on historical cropping patterns. Crop yields were based on annual average yields, as extracted from Reclamation Annual Summary Reports. Prices received for crops were obtained from the Economic Research Service, (ERS) of the United States Department of Agriculture (USDA). The price of corn silage was not available from the ERS, so a 5-year average of corn prices obtained from the Colorado Department of Agricultural Statistics was used. Representative farm expense inputs to the budget were taken primarily from extension farm budgets, machinery cost studies, discussions with farmers and others knowledgeable with agriculture in the area, and budgets used in previous studies. Farm production costs were obtained from the Colorado State University Extension Service, USDA Agricultural Price summaries, Doanes Reference Manual, and interviews with farmers in the District. The representative non-irrigated farm also consists of 550 acres; however, almost all of the land would be dedicated to rangeland.
The irrigation benefit analysis resulted in a difference in income between the irrigated and non-rrigated farms of about $134 per acre, or about $45 per acre-foot, based on an average water application rate of about 3.0 acre-foot per acre. Multiplying the benefits per acre-foot ($45) by projected year-by-year water deliveries results in a capitalized present worth value of about $11 million.
b. Municipal and Industrial Water Benefits - M&I water deliveries through Horsetooth Reservoir currently average about 69,500 acre-feet annually to East Slope metropolitan areas. This annual amount includes about 42,800 acre-feet of CBT Project water and 26,700 acre-feet of non-CBT Project water provided by the Windy Gap Project. However, if Horsetooth Reservoir was eliminated, the CBT Project would be reoperated to provide about 53,300 acre-feet of water for M&I purposes. This would include about 32,800 acre-feet of CBT Project and about 20,500 acre-feet of non-project (Windy Gap) water. Therefore, in effect, net M&I water currently attributable to the existence of Horsetooth Reservoir is about 16,200 acre-feet. This amount includes about 10,000 acre-feet of CBT Project water, and 6,200 acre-feet of non-project water.
Under Reclamation P&G provisions, the conceptual basis for the economic value of M&I water is society's willingness to pay for the water. Consistent with those provisions, M&I water benefits were estimated based on the market value of M&I water provided by the CBT Project. Discussions with Northern Colorado Water Conservancy District (NCWCD) and developers in the East Slope area established that shares of CBT Project M&I water are being traded by developers at a capital cost of about $20,000 per acre-foot, which is equivalent to an annual cost per acre-foot of about $1381 when amortized over a 50-year period at the Federal fiscal year 2000 interest rate of 6.625 percent. This is probably a conservative value because officials at NCWCD indicated the cost has been constantly increasing due to the active market created by the rapid population growth within the District.
Projected use of CBT Project M&I water was gradually increased to reflect continued transition of water use from irrigation to M&I use. The projected transition resulted in an increase in the annualized amount of CBT Project M&I water use attributable to Horsetooth Reservoir from a current average annual amount of 10,010 acre-feet to an annual amount of about 21,000 acre-feet at the end of the 50-year projection period. Based on projected year-by-year water deliveries and the annual value per acre-foot of $1381, the capitalized present value of annual CBT M&I benefits attributable to Horsetooth Reservoir is about $259 million.
Horsetooth Reservoir also allows average deliveries of 6,200 acre-feet of non-CBT (Windy Gap) M&I water annually. These additional water deliveries result in average annual benefits attributable to Horsetooth Reservoir of about $8.6 million. The capitalized present value of annual non-CBT water benefits attributable to Horsetooth Reservoir is $124 million.
Thus, M&I water benefits attributable to Horsetooth Reservoir total about $383 million.
c. Recreation and Fish and Wildlife Benefits - Horsetooth Reservoir has a total capacity of about 156,735 acre-feet, a surface area that normally averages about 2000 acres during prime recreation months, and a shoreline of about 14 miles. Recreation at the reservoir is administered by the Larimer County Parks and Open Lands Department (LCPD). Recreation developments include 4 campgrounds, lll campsites, 8 boat-launch ramps, a public marina, and a developed public swim beach. Recreation visitation rates at the reservoir were provided by LCPD. Recreation visitation has remained fairly constant at about 500,000 persons per year for the past three years, even though demand for recreation is increasing rapidly due to the burgeoning population in the area. The lack of growth in recreation visitation is explained by the fact that most recreation facilities are operating at full capacity during the prime recreation months. According to LCPD, visitation is divided approximately as follows: 40 percent camping, 40 percent boating, 10 percent fishing, and 10 percent all other activities. Primary sport fish include rainbow trout, crappie, small mouth bass, white bass, largemouth bass, and walleye.
In order to determine the net economic value of recreation benefits provided by the reservoir, it is necessary to take into consideration the potential for absorption of some of the recreation visitation by other recreation areas, in the absence of Horsetooth Reservoir. However, according to the LCPD, competing recreation areas are essentially already at full capacity during prime recreation months, so there is little or no opportunity for the substitution effect. This is a reflection of the rapidly growing demand for recreation in the area. Accordingly, recreation benefits were based on recent annual visitation data, with no reduction for the substitution effect. Calculation of annual benefits was based on consumer surplus values for a recreation visit as contained in the U.S. Forest Service (USFS) Publication RM-289 (1), and Colorado Water Resources Research Institute (CWRI) Technical Report no. 54.(2) The CWRI source was used for boating benefit estimates, because the USFS source does not contain values for that activity. The USFS benefit values for a recreation visit for each primary activity, which reflect 1986 price levels, were indexed to 2000 using the Consumer Price Index.
Computation of benefit values is summarized below.
| Camping | 200,000 visits | @ | $48.13 | per visit | $9,625,000 |
| Boating | 200,000 visits | @ | $27.44 | per visit | $5,488,200 |
| Fishing | 50,000 visits | @ | $52.09 | per visit | $2,604,400 |
| General | 50,000 | @ | $30.03 | per visit | $1,501,400 |
| Totals | 500,000 | $19,219,000 |
If any of the dams retaining the reservoir were breached, recreation at the reservoir would essentially be eliminated, because 93 percent of the water volume would be lost and 82 percent of the reservoir surface area would be lost. Along with the loss of water-based recreation, the aesthetic impacts would virtually eliminate all other recreation activities. However, in order to be conservative in the loss estimate, it was assumed that 82 percent of recreation visitation would be lost, consistent with the loss of surface area. Therefore, 82 percent of the recreation visitation, and thus benefits of about $15.8 million, are assumed to be attributable to the existence of the fully operational reservoir. Capitalized at 6.625 percent over 50 years, estimated recreation benefits over the life of the dam would be about $228 million.
1. D. W. McCollum, G. L. Peterson, J. R. Arnold, D. C. Markstrom, and D. M. Hellerstein, The Net Economic Value of Recreation on the National Forests: Twelve Types of Primary Activity Trips Across Nine Forest Service Regions, Research Paper RM-289. Fort Collins, CO: U. S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, February 1990, Pages 15 and 17.
2. R G. Walsh, D. M. Johnson, J. R. McKean, Review of Outdoor Recreation Economic Demand Studies with Nonmarket Benefit Estimates 1978-1988, Technical Report No. 54, Fort Collins Colorado, Colorado State University, Colorado Water Resources Research Institute, December 1988, Page 49.
d. Hydropower Benefits - As noted earlier, CBT Project water is diverted from the West Slope to the East Slope only if it can be put to beneficial irrigation or M&I use, and these trans-Continental Divide diversions are also routed through East Slope hydropower plants. Any reduction in the storage capacity of Horsetooth Reservoir to address dam safety issues would result in the reduction in deliveries of irrigation and M&I water which, in turn, would require reduced diversions from the West Slope. The reduced diversions would impact both the quantity and timing of power generation. Therefore, in order to determine the impacts of the various alternatives to address the dams safety issues on power benefits, it was necessary to estimate current CBT Project power generation and benefits.
CBT Project power generation has averaged about 752.1 million kilowatt-hours (KWH) annually. If Horsetooth Reservoir was eliminated, reduced diversions from the West Slope would result in CBT project generation being reduced to about 674.4 million KWH. Therefore, the net generation attributable to the existence of the reservoir is 77.7 million KWH. The economic value per KWH of generation was estimated based on the alternative costs of a thermal power plant, as provided for by provisions of the P&G's. Almost all of the thermal capacity now being built is gas or oil-fired combined-cycle or combustion turbine technology because of both relatively low costs and the flexibility it provides to meet a broad spectrum of capacity requirements. Therefore, power benefits were estimated based upon the costs of a gas-fired combined cycle thermal power plant. The computations resulted in a marginal or economic cost per kilowatt-hour of $.0656. Multiplying this cost by the net average annual CBT power generation attributable to the reservoir (77.7 million KWH) resulted in estimated annual power benefits of $5.1 million. The capitalized present worth of these benefits is about $73 million.
e. Flood Control Benefits - Flood control benefits were not quantified due to a lack of historical data. However, a fairly recent example of how the reservoir serves in that capacity is provided by events during the July 28, 1997, detailed in section II.B.