The Making of a Natural Sandy Beach: Have Rivers in Southern California Ever Been an Important Source of Sand?

April 10, 2006

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

It is a common belief that naturally flowing rivers in Southern California are a major source of beach sand, replenishing grains washed to sea by heavy surf and high tides. As a corollary, dams and other human activities changing the natural course of rivers have been seen as robbing beaches of new sand, contributing to beach erosion and intensifying the need for replenishing beaches through engineered beach nourishment projects.

Santa Margarita River

Santa Margarita River mouth in northern San Diego County, the site of planned Sea Grant-funded fieldwork to understand the fate of sand in river discharges. [Photo: CaliforniaCoastlines.org]

The long-standing canon of beach dynamics, that rivers supply beaches with sand, may be overstated and overly simplistic, says Escondido-resident Neal Driscoll, a professor in the Geosciences Research Division at Scripps Institution of Oceanography, who has California Sea Grant support to study sedimentation patterns in San Diego and Orange counties.

“I am not saying rivers are an unimportant source of sand,” Driscoll said. “I am saying they may not be a major source of sand.” As a result, dams and urbanization may not have altered the region's sand budget in the way, or to the degree, that researchers had previously imagined.

Past studies of beach processes led scientists to estimate that rivers bring as much as 90 percent of sand to beaches and that dams, therefore, cut off an equal amount of the coveted material. This estimate, Driscoll said, may be too high.

Driscoll is not alone in his theory that rivers have been ill cast as the lead player in maintaining sandy beaches. Coastal engineers at UC San Diego led by professor Scott Ashford, a resident of Encinitas, recently used laser-imaging techniques to create highly detailed digital maps of the shape of coastal bluffs in San Diego County. Analyses of the changes in the bluffs' shape over time let them compute the volume of material shed by the cliffs. Assuming all eroded cliff material was sand and went to beaches, coastal cliff erosion could supply more than half of all sand on some beaches in Southern California, they reported. That would make coastal bluff erosion the single most important natural sand source, and it would imply that sea walls, rip-rap and other hard structures built to halt bluff erosion contribute to narrowing of sandy beaches and might be doing so faster than thought.

This discovery, which is being more fully explored in an ongoing collaborative project with Sea Grant, is fully consistent with Driscoll's theory since its basic message is that coastal bluff erosion has been an overlooked sand source.

Graph

The plot shows the amount of water exiting through the mouth of the San Luis Rey River in northern San Diego County between 1929 and 1979. Flows are infrequent and correlated with periods of heavy rainfall. Most of the time, the river is dry. If it supplies beaches with sand, it must do so during these relatively infrequent deluges. Graphic: Jennifer Haas and Neal Driscoll, Scripps Institution of Oceanography

Driscoll is taking the science of sediment dispersal a step further by re-examining the role of rivers in supplying beaches with sand. "People have oversimplified many things about beach dynamics," he said. "I am going back to square one."

Rivers are akin to conveyor belts, he explained. They shuttle sediments to the coast. Think of the prominent granite boulders of inland North County being slowly scoured by water and the reddish clay soils of the scrublands melted away. A lot of sediment is transported, but this is not the same as saying this sediment ends up, let alone stays, on beaches, he said.

Instead Driscoll believes that the precious sand may be destined for the bottom of the sea. A compelling piece of evidence for this idea comes from stream gauge data along rivers in semi-arid climates such as Southern California, the flows through the San Luis Rey River mouth being a prime example. "Most of the time there is no water coming out of the San Luis Rey," Driscoll said. "But when it 'goes,' it goes big. If sand is coming from the river it has to be coming in huge pulses." [See Figure 1]
These sand pulses increase the density of river water, potentially making it more dense than seawater, which is normally heavier than freshwater because of its salt. This means that at the coast, the water coming out of the river might not float atop the saltwater and drop its sediments into shallow water, where waves and tides can return sand to beaches. Instead, the water-sand slurry may sink to the bottom and move to deeper water, bypassing the nearshore system, effectively eliminating the river’s ability to serve as an input of new sand.

"It is like having molasses move through the water," Driscoll said. "Something heavy sinks and moves along the bottom."

San Diego Bay Shipping Channel

This is an image of the shipping channel in San Diego Bay, created using the same acoustic techniques that will be used to map the sea floor off the Santa Margarita River in northern San Diego County. The dark blue shows the bottom of the shipping channel. Note the hard edges of the channel depicted by the two-tone, rugby stripelike, blue layer. The thin cuts across the channel are fiber optic cable lines. Graphic: Jennifer Haas and Neal Driscoll, Scripps Institution of Oceanography

Southern California is prone to molasses-like river discharges, known scientifically as gravity currents, because there is not much vegetation to grip the soil in place during heavy rains. When it rains hard, the water is very effective at picking up sediments. There is a lot of erosion, and because the rivers are flowing fast, a lot of sediment can stay suspended in the water column.

The theory that sediments carried by rivers might be bypassing the beach zone was first put forth by geologist Jonathan Warrick of the U.S. Geological Survey in Santa Cruz during studies of the Santa Clara River in Ventura County. He and colleagues who placed instruments on the continental shelf to acoustically and optically measure sediment concentrations and water velocities showed that the flows from the river did indeed sink to the sea bed. "We saw these gravity currents moving offshore from the river mouth," Warrick said. A gravity current is a highly concentrated flow of water and sediment. "It is analgous to a mud flow on land."

In a published paper on the research, Warrick hypothesized that the gravity currents observed at the Santa Clara River might be characteristic of river dynamics in general in Southern California, and that as a result much of the sediment load in the region's rivers might be deposited directly on the adjacent continental shelf, thus representing a "loss" of a potential sand source.

Warrick's field work, conducted in the winter of 2004, focused on documenting a gravity current in the region, at the time a first. The field studies did not attempt to examine the logical follow-up question for those interested in the understanding beach-building processes: How much sand is actually contained in these currents? "We proved these flows exist," Warrick said. "But, we don't know how much sand is within them. I can guarantee that there is a lot of mud. We don't know about the sand."

This is where Driscoll's work comes in, as he will soon be leading field experiments along the Santa Margarita River, which travels through Camp Pendleton and is the most unaltered river in the region. These experiments will seek to document whether sediments carried by the river are transported offshore and then, if relevant, to quantify the amount of sand in these bottom flows.

To do this, Driscoll and colleagues will first produce a detailed map of the ocean floor around the river mouth using an acoustic technique known as "swath mapping bathymetry," which is roughly akin to giving the seafloor a sonogram. A second technique known as "chirp seismic reflection" will be used to reconstruct an image of how the sediments are layered with depth. Think of taking a cross-section of a tree trunk to see its annual growth rings. [See Figure 2]
The idea is to re-measure the shape of the seafloor later, after a major storm, to see if sediments carried by the river are indeed accumulating at depth as theorized. If they are, the chirp seismic reflection data will make it possible to estimate the amount of sand in the newly formed sediment layer.

San Luis Rey River

San Luis Rey River in northern San Diego County. This river is in a heavily urbanized area. Scientists debate the effects of urbanization and dams on the ability of rivers to resupply beaches with sand.

In previous research, Driscoll and colleagues collected sediment cores, an exhaustive labor-intensive process, to map changes in the seabed off the mouth of the Eel River in Humboldt County. There, they were able to show that material discharged through the river’s mouth during a 1995 flood coated the nearby seafloor in a 10-centimeter layer of sediment. The sediments had the telltale mineral signature of having come from the river, and they were deposited at depths that made it unlikely waves and currents would sweep them to the beach anytime soon. Only a few percent of deposited material was sand, he said. Whether this is representative of sand percentages in Southern California remains to be seen.

"People have thought for so long that rivers must bring sand to beaches," said Warrick, who is familiar with Driscoll's research and interested in its results. "You look at a river. The water moves many meters a second. You can watch the plume from it go out to the ocean. You see the plume slow down really, really fast. You think the sand must fall out. We never thought about gravity currents. It is one of those things, where we opened our eyes and said 'Wow.' We sort of have a hangover trying to understand it."

As compelling as their theory and preliminary data are, Doug Inman, a research professor at Scripps Institution of Oceanography, who was among the first to propose that rivers have been the dominant sand source for the region's beaches, is firm in his belief that without natural river flows, wide beaches are destined to be a thing of the past.

"At the time (his work began in the late 1940s), we warned that all the dams were cutting off the sand supply," Inman, a La Jolla resident, said. "If you make studies now, you are missing the sand that would naturally come from all the tributaries. You can look at the sand that builds up behind dams, in all the catch basins and in all the small canyons, this all adds up. Also, everything covered by the hard cover of modern society – roads, homes, all the hard-scape – prevents sand from going into streams. All these take away from the sand that normally would go to the coast and beaches."
In regards to coastal bluff erosion, he said that beaches protect cliffs. "When you cut down the sand supply from rivers," he said, "you are, in fact, exposing more of the coast to wave erosion. It follows that cliff erosion must be greater than it was before. We all agree that cliff erosion is much more intense and important than it was 30 or 50 years ago."

"But, it does not matter whether cliff erosion or gravity currents occur," he said. "It is whether they occur in an amount and at a frequency that is relevant to the total sand budget." His belief is that the region, because of dams and urbanization, is running a sand deficit. Translation: beaches are losing sand and becoming narrower.

"Nobody has yet been proved that gravity currents are an important process, the normal state of things, either today or in the past," he said. "It needs to be checked carefully and shown that these gravity currents are what normally happens. Getting more data is important."

On this Driscoll agrees. "Theories come and go," he said. "That is science. Numerous researchers are examining how sediment is transported once it reaches the ocean. ... It shows the health of science that we are asking questions."