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Sand Starvation
Urban sprawl - Carmel Valley looking east towards Black Mountain. Photograph by Pat Masters. Image used with permission.
uman population growth has led to urban sprawl, hardened watersheds, and damming of rivers. The result is sand-starved beaches.
Since World War II, California's growth pattern has consumed vast tracts of land for low-density development. For example, from 1970 to 1990, the population of Los Angeles increased by 45 percent while the amount of developed land increased by 300 percent (Diamond, H. L. and Noonan, P. F., Land Use in America, 1996). San Diego County's population increased 5 fold since 1950 while Orange and Riverside counties increased 10 fold.
• San Diego Association of Governments
• U.S. Census Bureau
- Population of Counties by Decennial Census:
1900 to 1990
- California Quick Facts
In the past, up to 90% of natural sand supply for California beaches came from rivers and streams. Water runoff from a natural watershed transports a mixture of sand, silt, and clay to the coast. Silt and clay are then dissipated to deeper water while the sand remains to protect the beach.
Dam at Lake Hodges on the San Dieguito River. Photograph by Pat Masters. Image used with permission.
The hard cover of streets, parking lots, and structures reduces the amount of sediment produced by precipitation and runoff. Urban drainage systems are designed to control runoff and prevent the transport of sediment and pollutants to waterways and eventually to the ocean.
Now dams as well as urbanization have altered the natural watersheds. More than 480 dams have been built in California watersheds draining to the Pacific Ocean -- with 39 dams in the basins of the Oceanside littoral cell.
The littoral cells of southern California, from Point Conception to San Diego, are most impacted by dams: modern sediment supply from streams is reduced drastically from historical fluvial supply. In the Santa Barbara littoral cell, dams have reduced sediment flux from streams by 41%. The Silver Strand littoral cell has lost 72% of its natural sand supply!
Platform exposed at Solana Beach - January 2003. Photograph by Pat Masters. Image used with permission.
A close look at the Oceanside littoral cell in the period since World War II highlights the influence of climate and urbanization on sand flux to the coast. From 1943-1999, the San Diego region experienced tremendous population growth and watershed modifications, as well as periods of dry and wet climate. Dam construction peaked in the 1970s, but urban sprawl continues to reduce natural watershed area.
Changes in sediment flux, derived from U. S. Geological Survey monitoring, are given in the table below. Annual sediment fluxes are compared with estimates of natural fluxes that would have occurred in the absence of human modification. The "calculated annual sand flux" was determined by making a reasonable estimate that 20% of the measured sediment flux is sand. Our estimate of "natural annual sand flux" accounts for basin modifications that increase both the area behind dams and the amount of incorporated area over time. Time-averaged decreases in erodible basin area were used to back-calculate the natural sand fluxes expected under unmodified watershed conditions.
Comparison of measured and natural sand fluxes in the Oceanside littoral cell.
(in thousands of cubic meters per year)
| Basin | Calculated annual sand flux | Natural annual sand flux | Ratio Calcluated / Natural sand flux | ||||||
| Dry (1943- 1977) |
Wet 1978- 1998) |
Total (1943- 1998) |
Dry (1943- 1977) |
Wet (1978- 1998) |
Total (1943- 1998) |
Dry (1943- 1977) |
Wet (1978- 1998) |
Total (1943- 1998) |
|
San Juan Creek |
1.28 |
15.7 |
6.67 |
1.50 |
25.7 |
10.6 |
0.85 |
0.61 |
0.63 |
San Mateo/ Onofre Creek |
1.86 | 22.9 | 9.75 | 1.90 | 24.6 | 10.4 | 0.98 | 0.93 | 0.94 |
| Santa Margarita River |
5.40 | 25.8 | 13.0 | 9.26 | 59.5 | 28.1 | 0.58 | 0.43 | 0.46 |
| San Luis Rey River |
3.98 | 170 | 66.4 | 6.71 | 338 | 131 | 0.59 | 0.50 | 0.51 |
| Lagoon |
2.11 | 19.0 | 8.42 | 3.21 | 70.7 | 28.6 | 0.66 | 0.27 | 0.30 |
| San Dieguito River |
3.45 | 31.0 | 13.8 | 8.14 | 116 | 48.5 | 0.42 | 0.27 | 0.28 |
| Peñasquitos Creek |
0.986 | 8.86 | 3.94 | 1.55 | 33.3 | 13.5 | 0.64 | 0.27 | 0.29 |
Totals for Oceanside Cell |
19.1 |
293 |
122 |
32.4 |
668 |
271 |
0.59 |
0.44 |
0.45 |
Important points we can learn from this table:
- Sand flux in the Oceanside Cell is highly correlated with wet climate periods: the monitored amount of sand flux to the coast during the recent wet period is 15 times the amount transported during the previous dry period.
- One river -- the San Luis Rey -- accounts for about 50% of the total sand flux in the Oceanside littoral cell.
- The San Luis Rey is an important sand source only in the wet period.
- The ratio of monitored/natural sand flux for the dry period (0.59) is higher than the ratio for the wet period (0.44) because the dry period occurred earlier when there were fewer watershed modifications.
- Small basins (San Juan, Lagoon, Los Penasquitos) have relatively high sand yields, an observation that is consistent with many other monitored streams in semi-arid regions of the world.
Complexities The two largest streams in the Oceanside littoral cell -- the Santa Margarita and the San Luis Rey -- illustrate the complexities of sediment transport due to geological settings and human interventions.
Although it is the only "wild" river remaining in southern California, the Santa Margarita transports less sand than the San Luis Rey because for most of its length the Santa Margarita riverbed is stabilized by granite bedrock and cannot erode. In contrast, the San Luis Rey riverbed is alluvial and easily eroded.
Sand mining in streambeds causes a net loss of sand to the littoral cell in the long-term, but the small sand mines operated in the San Luis Rey riverbed are increasing sand supply to the coast for the present. By creating irregularities in the shape of the streambed, the mines accelerate erosion. There are no sand mines anywhere on the Santa Margarita.
In addition, the lower San Luis Rey River valley has been channelized by the U.S. Army Corps of Engineers. These levees prevent deposition in the alluvial plain and shunt the sand directly to the ocean.
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Last modifed Thursday September 23, 2004