By Joseph Hill
The early history of San Diego County revolved around water. The dry summers and years of low rainfall severely restricted the amount of water available for drinking, agriculture and other important activities. As a result, dams were constructed across major rivers to collect winter runoff from the seasonal rains for use during the dry summers. This proved to be a viable plan for the typical yearly water cycle. However, extreme flood years caused devastation in the rivers and creeks, even destroying parts of the early Old Town in the process. Water supply dams are closely related to flooding and flood control; they are two sides of a coin. The history of floods, flood control facilities, and flood management programs in early San Diego is presented in this article.
San Diego County covers about 4,200 square miles. About 60 miles from south to north and about 70 miles across, it is larger than some eastern states. The terrain, from the coast in the west, rises eastward through the foothills, over the mountains, then descends into the desert. Most of the storm systems that affect the coastal slope come from the Pacific Ocean and head eastward over the mountains. The rising ground level pushes the moisture-laden air up, causing increasing amounts of precipitation as the ground elevation increases. By the time the storms pass the mountain divide, most of the moisture has been removed, and the expanding air mass that passes over the desert provides little precipitation. Average annual precipitation is 10 inches for the coast, 15 inches for the foothills, 30 inches for the mountains, and 5 inches for the desert. The small amount and the wide variation of rain present a problem for the County. The 10 inches of rain along the coast creates a near desert-like climate. Most of the rain falls in winter, with no precipitation in the summer months. The rivers and creeks become dry, and there is very little water to sustain life in the summer and much of the fall. The logical response in this environment was to construct dams in the foothills to collect winter runoff from the higher elevations of the coastal slope and to prevent the water from infiltrating into the groundwater basins, then seeping into the ocean.
In a semi-arid environment, dams serve two purposes. Water for such needs as drinking, cooking, and agriculture are essential and require the dedication of resources and the creation of infrastructure to allow an adequate annual supply. Additionally, dams provide flood control in an area prone to unpredictable rainfall. A basic relationship exits between the amount of water available and the number of people in an area. At the time the missionaries operated the Mission in San Diego, the native population consisted of several groups that lived in the area of San Diego County. To some extent, these people migrated toward the coast during periods of ample rain and retreated toward the mountains when the weather was dry for an extended time. Estimates place the native population in the County just prior to 1850 at 2,000 to 5,000 people. This provides an indication of the population that could be sustained without manmade water resource facilities.
The following table relates calendar years to census data and reservoir capacity in acre feet and provides a yardstick for considering the need for storage and water transportation facilities developed in San Diego County. The ratio of acre feet per Capita provides a comparison between reservoir storage and population. Of course, the reservoirs had to be filled by runoff from seasonal rains and evaporation during hot summers reduced the amount of available water. A series of dry years could deplete the water stored in the reservoirs.
SAN DIEGO COUNTY POPULATION and RESERVOIR CAPACITY
|SAN DIEGO CITY||SAN DIEGO COUNTY|
|Year||Population||Res. AF||AF/Cap.||Population||Res. AF||AF/Cap.|
a Actual figure from special census by U. S. Bureau of the Census, taken in March, 1957
c Including La Playa
*Water supply was supplemented by importing water from the Colorado River
The San Diego Mission and the Presidio were established in 1769, near the present site of Old Town. The Mission was moved up the river about three miles to its current location in 1774, in part to be near a reliable water source. The first complete and accurate map became available after the survey by Juan Pantoja y Arriaga in 1782. This map indicated the location of the Mission on the river, whose outlet was just south of the Presidio and Mission Bay (then called False Bay). Because of the scarcity of water, the missionaries found it necessary to construct a dam at Mission Gorge. Completed in 1816, it was 12 feet high and carried water to the Mission by way of a flume. In 1821, Mexico took control of the missions from Spain, and the missions were subsequently secularized and no longer supported.
The 1853 map by U. S. Army Lieutenant George H. Derby shows the San Diego River next to Old San Diego, now Old Town. A delta is identified downstream of Old Town and next to San Diego Bay. An 1856 sketch is consistent with the 1853 map. A dry riverbed can be seen next to Old Town, and the patterns of previous river flows identify the delta shown on the map. Note that the riverbed disappears under an area that is fenced on the right side of the picture. In 1862, the dry riverbed disappeared, at least for a few months. Torrential rains and floods washed away the east wing of Casa Bandini, though much of the structure survived.
Derby is known for more than his map. The city fathers had been concerned about the river flooding Old Town and filling the harbor in San Diego Bay with silt. Derby, an engineer, was assigned the task of designing and constructing protective measures for the city. He designed a dike from the hill north of Old Town, in a westerly direction, to the higher ground north of Point Loma. However, the structure was not adequately protected from the high velocity of floodwater, and the dike broke in the flood of 1862. By 1864, drought conditions prevailed again. In 1875, Congress appropriated $80,000 to rebuild Derby’s dike. This time it was properly designed and constructed to resist the flooding of the river. It withstood the floods of 1884 and 1916.
In 1889, the Cuyamaca Dam was constructed on Boulder Creek in the Cuyamaca Mountains, along with a 45-mile long flume. The flume went through eight tunnels and over 315 trestles. The earth fill dam has a rock face and is 41.5 feet high, including 6.5 feet above the spillway level. The total crest length is 665 feet. While this was a major accomplishment, it was far from sufficient. By the turn of the century, six reservoirs were in use and diversion structures and canals were providing water for San Diego County.
Several types of dams were used to create the reservoirs/lakes in an area that had no permanent fresh water lakes or ponds. The technology for design and construction of these dams, pipelines, and canals was relatively new, and there were no long-term records of rain, river flow, and infiltration to use as a basis for design. One type of dam construction used was the gravity arch. This was a new development in concrete dam construction in the late nineteenth century. It relies on the weight of the concrete to hold it in place and the arch effect directs the water force into the rock abutment on each side of the dam. Sweetwater Dam was the highest structure of this type when it was completed in 1888. A hydraulic fill dam was also an innovative method at that time. It was difficult to transport fill long distances to place it at the core of an earthfill dam. The problem was overcome by pumping water and silt to the dam site and creating a pond, then allowing the water to seep into the ground, leaving the silt in a compacted core for the dam. La Mesa Dam was constructed in this way. A third type of dam is rock with steel core. Several dams of this type were constructed in San Diego County. The method consisted of digging and/or blasting a trench at the center of the dam from the top of one abutment, through the bottom of the streambed, and up to the top of the other abutment. The trench is filled with concrete and the bottom of a steel core is cast into the concrete. The steel core is raised like a curtain to the elevation of the dam crest. Each side of the core is filled with rock to provide strength for resistance of the water force. The steel core limits seepage through the dam. Details of specific dams are discussed below.
Sweetwater Dam started as a 60-foot high structure. However, it was designed so that it could be expanded in height. During construction, additional analysis of the reservoir area showed that raising the dam to 90 feet would store five times more water in the reservoir. So, as the 60-foot high structure was nearing completion in January 1887, the decision was made to continue work for another 16 months to raise the dam to 90 feet. It was finally completed in April 1888, and it provided a valuable water supply for the area. Then in 1895, a rain of six inches in a 24-hour period created a flood greater than the designers had anticipated. There was no precedent for expecting that much rain and runoff in a short period. The result was that Sweetwater Dam was overtopped for a period of 40 hours, with the highest reservoir level 22 inches over the elevation of the parapet. Sweetwater Dam remained stable during this event. The cascading water caused erosion downstream of the structure and washed away some of the pipeline and other facilities, but did not adversely affect the dam. Following this flood, the parapet was repaired and raised two feet, but with 200 feet in the middle left as an overflow weir or spillway. An additional spillway was added on one side of the dam, to convey flood flow through the dam, rather than have it pass over the dam. An apron was added below the spillway to convey floodwater safely past the structure. The walls of the canyon below the dam were protected with concrete and steel beams. A 15-foot high wall was also constructed 50 feet downstream from the dam as a water cushion basin for flow going through the 200 foot overflow spillway. This basin would absorb the impact of water falling from a height of about 90 feet over the dam..
Escondido Lake, Dam and Canal were constructed to provide water to the Escondido area, which needed irrigation for crops in the valley. The relatively small watershed (8 square miles) is not large enough to provide sufficient runoff, so a canal 15.6 miles long connects the reservoir to a diversion structure on the San Luis Rey River. The conduit includes 67,287 feet of excavated channel, 14,142 feet of flume (wood channel supported by posts on the slopes of a rugged canyon) and 806 feet of tunnel.
This type of rock fill dam had given satisfactory service for many years in the mining regions of northern California. The upstream face of the dam consisted of two layers of wood planks. The joints were calked and smeared with a hot asphalt substance. Although the dam leaked (100,000 gallons per day) it was considered stable and the leakage could be used for irrigation during dry periods.
The United States Geological Society took note of the construction of the La Mesa Reservoir and Dam in a 1898 publication:
A novel and exceedingly interesting type of dam is that constructed in the spring of 1895 by the San Diego Flume Company for the purpose of storing the flood water of San Diego River in winter, as well as to provide a means of catching … the water at the lower end of the Flume.
This shows the interest the people of that time had in this structure. While the reservoir was not directly on the San Diego River, a canal brought water to the reservoir from the river. Since the Cuyamaca transported water to the San Diego River, any excess water not used for drinking, irrigation, or other needs was stored by the dam’s reservoir. During construction, hydraulic fill was conveyed into the area through a pipe, carrying a mixture of very fine soil and water. The water kept the soil in suspension. The pipe discharged the water containing the soil into a pond in the middle of the dam. Since this was a relatively new method of construction, people were also interested in this project for that reason.
Morena Lake and Dam, a rock fill dam structure on Cottonwood Creek, has a watershed that is in the higher elevations of San Diego County (the reservoir site is about 3,100 feet above sea level) and therefore has a relatively high annual precipitation. It was estimated that the reservoir would fill every year. Water could be discharged at a controlled rate into the creek and picked up by the Barrett Diversion Dam, a dam and canal system, built to direct water from Cottonwood Creek through a canal, 12 miles in length, to Lower Otay Lake.
Lower Otay Lake and Dam is a rock fill structure with a massive masonry pour as a base for the steel plates. The steel plates, riveted together, acted as a diaphragm to control seepage and can be seen at the middle of the dam. A pyramid formed by the steel members during construction was used as a crane to move rock, steel and concrete to their proper location in the structure. Explosives were used to break rock into smaller units for use in the dam. As an example of the effect of blasting, records show that 100,000 pounds of powder loosened 150,000 tons of rock.
The following table lists major facilites that supplied water in the late 19th century.
SAN DIEGO RESERVOIRS (19th Century)
|Type of Dam||Comments|
|Cuyamaca||1889||11||35||11,400||Earth/rock||Storage for Canal|
|Escondido||1887-94||8||75||3,500||Rock Fill||Lake Wohlford|
|La Mesa||1895||5||65||1,400||Hydraulic Fill|
|Barrett Div.||1896+/-||250||60||590||Rock Fill||Small structure, diverted flow to Canal|
|Lower Otay||1897||99||125||38,300||Rock Fill|
* Total area before Loveland Dam
At the turn of the twentieth century, a population of about 18,000 people were being supplied with water from a system that captured a major part of the runoff from rain in the area. However, San Diego was still dependent on local rain to supply the reservoirs with water, and drought conditions prevailed in the first 15 years of the century. City fathers offered a local rainmaker named Charles Hatfield $10,000 if he could cause enough rain to fill Morena Reservoir. The first day of January 1916, the dry weather continued. Hatfield had built towers near Morena Reservoir and was releasing strange and secret vapors into the atmosphere. The need for water was becoming more desperate each day. On January 14, 1916, a massive storm rolled in from the Pacific, and for the next six days, most of Southern California was deluged. Old Town received more than four inches of rain, and Cuyamaca received 18 inches, amounts that equaled nearly one half of their usual annual rainfall. Every river and creek was choked with floodwater, and Lake Morena was full. No doubt Hatfield was ecstatic, probably celebrating his success that had saved San Diego. The rain ceased on January 19th, and the area began to dry out. Then, just a few days later, the skies clouded again, the wind blew from the southeast, and a second deluge began, which pounded down sheets of rain, day after day. The storm added another three inches of rain at Old Town and more than 14 inches at Cuyamaca. The already saturated watershed retained little of the rain, and raging torrents of water raced down the rivers and creeks, destroying nearly every object in its path. One thing that remained structurally sound was Derby’s dike. It prevented the onrushing river from devastating Old Town and filling San Diego Bay with tons of sediment. Morena Reservoir has a large watershed, and the dam survived the large flood runoff without major damage. At Sweetwater Dam, the spillway was not large enough, and the floodwater overtopped the dam and then washed out a large section of the south abutment. The dam had been constructed with enough structural stability, so that it did not wash out. When the dam was later repaired, a second spillway was incorporated in the south abutment.
Otay Dam, however, was a total loss. The spillway did not have sufficient capacity for the flood flows and the dam was overtopped. Water filled the observation shafts on the downstream side of the steel core, and the pressure blew out the rock that provided the structural stability. The steel core swung out like a gate, releasing the full depth of water, which created a flood wave in the canyon of gigantic proportions. The dam was about 130 feet high, and the depth of the wave in the canyon a short distance below the dam site was about 100 feet high. The force of the water was so great that it stripped every bit of vegetation from the canyon walls, leaving a clear trace of the wave crest. The lower canyon is much wider, and the wave height decreased to approximately 20 feet, which was still devastating to the people living in the valley below. Every structure was destroyed and many people lost their lives. At the end of the flood, San Diegans found that every bridge over every river or creek was impassible, and the only way to travel to Los Angeles was by boat.
After the flood, Hatfield was nowhere to be found. Some time later, he appeared at the City Council to claim his $10,000. However, the city attorney determined that either Hatfield was responsible for the rain, or he was not. If he collected the $10,000 for filling Morena Reservoir, he was also responsible for the flood damage. Hatfield did not collect the money, and he disappeared from the area.
Subsequent to the 1916 flood, damage or destroyed structures were repaired and additional reservoirs were needed. The table above lists construction projects at reservoir sites throughout the county.
Major floods occurred again in 1927, 1937, 1978 and in 1980. None were of the magnitude of the 1916 event, and the additional reservoirs reduced the flood flows through the coastal areas. Records show the flows during the 1980 flood at three reservoirs, El Capitan and San Vicente in the San Diego River watershed and at Hodges on the San Dieguito River, indicating the degree to which the dams prevented flood damage. El Capitan Dam reduced the peak flow from (at least) 40,000 cubic feet per second (cfs) to 0. All the floodwater was stored in the reservoir and did not spill until several day later, when the sun was shinning. San Vicente Dam reduced the peak flow from 11,500 cfs to 6,000, almost a 50% reduction. The recorded flow downstream at Mission Gorge was 9,400 cfs, and the flow at Mission Valley was probably slightly higher. If the two reservoirs had not reduced the flows, the flow through Mission Valley would have been of the order of 50,000 cfs instead of about 10,000cfs.
The San Dieguito reservoir reduced the flow from 28,000 to 22,000 cfs. Sutherland dam and reservoir also reduced the peak flow, so the 28,000 cfs would have been higher without that dam. Hodges Dam spilled in 1980, and the Race Track at the lower end of the San Dieguito River flooded. The 1916 flood at this location had a flow of 72,100 cfs.
What about Sweetwater Dam in 1980? The spillway on the north bank that had washed out in 1916 functioned perfectly in 1980. All of the dams in San Diego County have had a dramatic impact on both water supply and flood control for the people of San Diego.