Deltas develop at the mouths of large rivers all over the world, but we seem to pay scant attention to them even though they play a vital role in our ecosystem. In this story we will explore how deltas are formed and their key environmental functions. We will also explore the Sacramento River-San Joaquin River Delta, the unique inverted river delta near the middle of California, just east of San Francisco Bay.
So what is a delta and how does it happen? A river often creates a delta at the spot where the river enters the sea or alarge lake because, as the river reaches the shore, the speed of its water flow slows way down. At the shore, the river’s water is no longer briskly moving, confined between its banks, but instead it spreads out into the sea in the shape of a fan and becomes just a part of the much larger volume of water that is the sea. For a short distance the momentum of the river water pushes it and anything it is carrying, like sediment, out into the sea, fanning out from the entry point, but after a short distance the river commingles with the sea and becomes indistinguishable from it. The sediment in the river water, no longer carried along by the river’s current, settles to the bottom. Eventually the accumulation ofsediment creates a fan-shaped shallow area at the mouth of the river. This is especially true where the sea near the river mouth is itself shallow. Aquatic plants begin to grow in the sediment. They build up the height of the alluvial fan, and they capture sediment and debris from the river water flowing over and around the growing fan. The alluvial fan, or as we call it, a “delta,” gets larger and larger over time as the river brings down more sediment.
Pic 1: Nile River Delta From Space
As the delta grows the river tends to separate into multiple smaller streams flowing individually through the low-lying fan. Each stream builds its own miniature fan at its mouth-the point at which that individual stream flows into the sea. The miniature deltas grow together around the outside of the existing delta and the overall delta grows even larger. Chance plays a role, too. If one of the streams happens to carry more of the river’s water, or it happens to traverse a shallower part of the edge of the sea bordering the delta, the delta will extend in that direction more than in others and some what lose its fan shape. When this happens, the delta is called a deltaic lobe type delta. Nevertheless, overall, most deltas are shaped more or less like a triangle and were named “deltas” because of that shape. The symbol for the Greek letter “D” is an equilateral triangle, which is, of course, fan shaped. In English the Greek letter “D” is called “Delta.”
Deltas are important to our ecosystems far out of proportion to their modestappearance. Like littoral salt marshes,the waters of these shallow territories lying at the boundary between the freshwater rivers and the salt water seas are some of the most fecund places onEarth. Life thrives in their shallow waters, nourished by organic matter brought down by the rivers and powered bythe sunlight that penetrates the shallow wasters. Both plants and animals abound and come in all sizes. Small animals like glassshrimp, fiddler crabs, worms, snails, clams, oysters, and mussels are complemented by a wide variety of fish, birds, snakes, and small mammals likerodents and rabbits. Even smaller than small animals are the insects, algae, plankton and diatoms on which the small animals feed, starting the upward climb of the food chain. These abound in the shallow, comparatively warm waters of the delta. Salt marsh grasses, eel grass, cord grass, saltwort, sedges, reeds, rushes, pond and spider lilies, and shrubs like Marsh Elder and Grounsel Tree rapidly develop.
Pic 2: Delta Plants
Pic 3: Reeds Grow In Deltas
Pic 4: Many of the animals listed above breed in the salt marsh waters of the growing delta, and their young grow up in the protection of the aquatic plants on the delta’s alluvial fan before entering the deeper ocean. For example, the shrimp industry of Louisiana depends heavily on the continued breeding of juvenile shrimp in the shallow delta waters of the Mississippi River.
The delta of the Mississippi River in Louisiana is home not only to a huge fishing industry, but also to an entire American sub-culture: the Cajuns, who come equipped with their own language, music and social customs. Damming of the Mississippi and its tributaries has reduced water flow and jeopardizes the continued viability of the delta population as well as its ecology.
Other deltas support huge populations. Therich soil of the Nile River Delta in Egypt has been farmed for perhaps 5,000 years and today is occupied by almost 40 million people. The construction of the Aswan High Dam on theupper reaches of the Nile has sharply decreased the volume of sediment flowing to the delta on river floods and the delta is actually growing a bit smaller each year as a result of erosion by the Mediterranean Sea. The Ganges/Brahmaputra Rivers combination delta is the world’s largest delta and comprises most of Bangladesh and a significant part of India. It is intensely farmed and occupied. Damming of the upstream reaches of these two rivers has jeopardized the stability of the delta, and the loss of fresh water supplies is adversely impacting the huge population of that delta region. The delta of the Yangtze River in China is an incredibly fertile agricultural area, feeding a huge part of the population ofChina. It is also a booming industrial and residential area, home to 80 million people and a skyrocketing manufacturing economy. It remains to beseen if the two can survive side by side on this delta. These problems, which are popping up around the world, emphasize the importance of deltas in our ecosystems and the need for mankind to guard against their loss.
Pic 6: In the Ganges Delta
Pic 8: The Ganges Delta encompasses huge urban environments as well as agricultural and wet lands areas.
Pic 9: The Earth is not alone in having deltas. Even Mars appears to have a delta formed by water.
Sofar we have explored how “normal” river deltas are formed and how they affect our ecosystems. Now let’s explore“un-normal” deltas. As interesting as normal river deltas are, another, very rare type of river delta, is even more fascinating and sheds even more light on the threats to our culture and economy that the loss of river deltas makes. A few of these un-normal river deltas have existed in the distant past, but,today, there is only one of them of significant size anywhere in the whole world. These river deltas are called “inverted” deltas. Instead of one river flowing from a single narrow channel and branching out into multiple streams upon entering the sea, inverted deltas form where multiple rivers streams come together in one confined area and flow into the sea through a single, narrowed channel. A delta forms where the multiple rivers come together, upstream of the narrow channel. Because the shape of this kind of delta seems to be upside down compared to the normal delta, this type of delta is called “inverted.” Pic 10, above, is a composite satellite image of an inverted delta with the rivers and bays color enhanced.
The inverted delta forms in much the same way as a normal delta. Multiple rivers moving rapidly carry sedimentdown from their sources, but because of the constriction on the flow of water caused by the single narrow channel of discharge the water slows down. It’s like all the fans at a sports event leaving at the same time through a single gate of a big arena. Everyone hurries down to the main floor and moves toward the gate, but, near the gate, the pace slows and becomes a crawl as the fans make their way to and through a few at a time. In the geologic world the rivers trapped behind the choke point of the strait are forced to spread out as they all try to squeeze out through the narrow channel at the same time. The slow down causes the sediment in their water to settle out in just the same way as it settles out of a river flowing through a “normal” delta as that river water slows down upon mingling with the sea. The inverted delta rivers’ water, minus most of its sediment, flows through the strait and out to sea. The inverted delta forms from the deposited sediment; plants take hold and grow, trapping more sediment, and the inverted delta grows just like a normal delta-an upside down fan. And, just as with a normal delta, plants, water creatures and land animals develop; farmers take advantage of the rich alluvial soil; and local populations grow.
There is only one large scale inverted river delta in the world today. Pic 11 is the Sacramento River-SanJoaquin River Delta.
At the north eastern edge of San Francisco Bay lies a narrow river channel throughthe California Coast Range Mountains. The channel, called the Carquinez Strait, is barely 3,000 feet wide. Through that channel, into San Francisco Bay, and then through the Golden Gate into the Pacific Ocean, billions of gallons of water empty daily, collected by the great river systems of California’s huge Central Valley. The Sacramento River from the north end of the valley, the San Joaquin River from the south end and their many tributaries collect rain and snow melt runoff from virtually the entire western flank of the Sierra Nevada mountains, the southern reaches of the Cascade Range, and the eastern flanks of those parts of the Pacific Coast Range that border the valley. All told, about half of all of California’s runoff from rain and snow melt pass through the inverted Delta and the 3,000 foot wide Carquinez Strait.
The map of the Delta's rivers and streams show how complex and interwoven the relationship between the low lying land of the Delta and its many water flows.
The Delta formed as the result of two major geological events: First, the California Central Valley formed as a level submarine shelf depressed by the subduction of a tectonic plate called the Farallon Plate under the North American Plate. (More detail about this is in the story “The Making Of California-Plate Tectonics vs. Mother Nature’s Weather And Erosion,Part 1: Panoche Road.”) With the rise of the California Coast Range Mountains, formed by the scraping action of the North American Plate over the subducting Farallon Plate and then Pacific Plate, from time to time, the Central Valley was cut off from an easy outlet to the sea. But rain and snow melt run off eventually won out over rock, and a outlet developed through the Carquinez Strait. Even so, for thousands of years large parts of the Central Valley were flooded during the spring as snow from the mountains melted. This vigorous runoff and the eroded soil carried in it promoted delta building. Over the ages the rivers carried sediment down to the area east of the Carquinez Strait, and the huge estuary we know today as the Sacramento-San Joaquin inverted Delta formed.
The Delta covers about 1,100 square miles (2,800 square kilometers); although there are towns in the Delta and major cities adjacent to it, the Delta is largely agricultural. Starting in the 1860s, hundreds of miles of levees were built around so called “islands” in the Delta, which were really shallow swamp land. The labor was done in large part by Chinese workers who came to California for the Gold Rush and the construction of the railroads. Mostly by hand, they built levees that enclosed about 70 of the larger agricultural tracts or islands. Water was pumped out of the islands and the fertile soil exposed, and a thriving agricultural economy has developed in the Delta, producing perhaps a half-billion dollars worth of crops each year.
Pic 12: A typical Delta Levee. The "island" to the right is 10 to 15 feet below the level of water in the river on the left.
Pic 13: The Delta's "islands" and levee system.
Unfortunately, the removal of the water exposed the fragile peat moss which underlay themarshes. The peat moss decomposed and shrank. The levees subsided, and the process necessitated building the levees higher and higher. The instability of the higher levees on alluvial soil proved and continues to be a danger to the Delta, its agriculture and water. The problem was exacerbated by huge quantities of sediment washed down from hydraulic mining operations inthe Sierra Nevada in the years before 1885. Fortunately, these were eventually stopped by court action in the Sawyer decision, one of the first cases in which a court acted to protect the environment, but the subsidence persists and the levees are constantly being raised, leading to a greater and greater difference between sea level and the surface of the islands. Today, almost the entire Delta lies ten or more feet below average sea level.
Pic 14: Several levees have broken and the rivers flooded the land behind them. Twelve thousand acres were flooded in 2004 near Stockton. In 1928 an asparagus farm was flooded by a levee failure subsequently called “Big Break.” The Franks Tract and the Jones Tract were flooded and Bethel Island partially flooded.
Even more worrisome is the possibility of a major levee break resulting from a huge storm. History and geology tell us that devastating flooding of the Central Valley does happen from time to time. In 1862 newly-elected Governor Leland Stanford attended his inauguration ceremony by row boat and quickly moved the California state capital to San Francisco until the waters subsided. Geological records show that major flooding is a periodic occurrence, and several geologists have predicted a two out of three chance of catastrophic flooding of the Sacramento area in the next fifty years. But so far the ongoing debate of the controversy in the California government about what to do with the system of levees has produced no comprehensive plan of action.
The Delta conundrum results from the incredible usefulness of the Delta. Its fecund agricultural land produces significant crops from rice and asparagus to grapes, citrus fruit and turf and contributes billions to California’s economy.
Pic 14: More importantly, the Delta is the source of much of Central and Southern California’s domestic and agricultural water supply. TheDelta is the heart of a huge north-to-south water-delivery system. Two giant pumping stations at the side of the Delta remove millions of acre-feet of fresh water from the Delta and send it south in canals and the California Aqueduct. Most of the water is used for agriculture in the Central Valley, which would be a largely unproductive dry plain without it, and the remainder goes to the continually growing population of Southern California.
Pic 15: The California Aqueduct snaking along the western side of the Central Valley
although the San Jose area receives some water from the Delta, the rest of the
San Francisco Bay Area gets very little, if any. San Francisco relies exclusively on the Hetch
Hetchy reservoir in the Sierra Nevada and on local rain runoff on the San
Francisco Peninsula. Marin County relies
on local rain runoff and some water from the Russian River Basin in Sonoma
County. The San Francisco East Bay
depends on reservoirs in the Central Sierra and local rain runoff. The water that falls as snow and rain in
Northern California is siphoned out of the Delta and pumped southward to
agriculture in the southern Central Valley and over the mountains at the foot
of the Central Valley to the Los Angeles Area.
The Delta water conundrum is three fold. First, not too far in the future, there will not be enough water in California to go around. Continued urban population growth and increasingly intense agriculture mean there will be no surplus, and no surplus means serious political and financial competition for the water. Second, removing more fresh water from the Delta will allow salt water from San Francisco Bay to intrude into the Delta. The salt water that will flow into the Delta from San Francisco Bay as a result of lessened outflows of fresh water will change the ecology of the Delta dramatically, and that will jeopardize fresh water supplies in the long run. In the shortrun it has already caused salinity problems in the water pumped south. Third, substantial breeding populations of fish, such as salmon, bass and smelt either spawn in the Delta or must swim upstream through the Delta to breed. The pumps that lift the water out of the Delta and send it on its long journey south already kill huge numbers of fish each year. A report prepared for the Sacramento County Sanitation District shows that at least 8,966,976 splittail, 35,556 chinook salmon, 430,289 striped bass, 54,412 large mouth bass,69,383 bluegill, 76,570 white catfish, 28,301 channel catfish, 233,174 threadfin shad, 264,171 American shad, 1,642 steelhead and 51 of the endangered Delta smelt were killed by the pumping system. Salinity changes in the water also confuse the migration process of other fish, preventing breeding runs of sufficient size to maintain a breeding population. Balancing all of these needs is a scientifically tricky and politically monumental process. If we do not manage to keep the Delta a living and functional ecological environment, we will jeopardize the entire State. Click here for the full report.
The competition for water is not the sole environmental pressure on the Delta. Other players have appeared on the scene. The Delta’s geological configuration means it is now a source of wind energy and, perhaps soon, tidal energy. Huge wind turbines already dot the Delta near the Carquinez Straight, and more are to come. The Delta may lie at the mouth of a funnel where many rivers come together from the east to escape westward into the Pacific through a small gap in the Coast Range Mountains, but that very same small gap focuses the strong winds blowing east from the Pacific Ocean into the area of the Delta just east of the strait. These steady, relatively high-speed winds are a reliable source of wind energy, which would be jeopardized by flooding. By the same token, the high speed/high volume water flow out through the strait may be a future source of clean electricity.
Any solution to the California Delta water problem will be a complex political,ecological and economic effort, but there is one thing we can do in the meantime: extensive conservation efforts. Farmers will need to invest in more efficient ways to irrigate in order to replace current practices like field flooding and continual sprinkler irrigation. Users of the water will need to learn to use less by more drought-tolerant planting and recycling, such as using gray water for irrigation.
Pic 16: If you have a chance to check out the Delta in person, you might also like to visit several interesting locales. TheHazel-Atlas Sand Mine near the Black Diamond Regional Preserve near Antioch on the on the Carquinez Straight is the location of the mining of sand-that’s right-high quality sand, which was used until about 1940 to make glass in San Francisco.
Pic 17: White Hazel Atlas Glass Bowl
Pic 18: Hazel Atlas Green Plate
Pic 19: Coal was mined nearby at the Black Diamond and other mines.
The town of Walnut Grove, established in 1850 by John W. Sharp, is one of the earliest settlements along the Sacramento River. It became the region’s agricultural shipping depot with a river front access to the Sacramento River and, consequently, to the San Francisco Bay and the City of Sacramento. Chinese immigrants built most of Walnut Grove, but when a fire destroyed the town’s Chinese residential area in 1916, many of the Chinese moved to a new town recently established nearby called Locke. Locke persists today in a semi ruinous state, but the 100-year-old buildings are still charming and invite a slow wander through the two block long town. The economic driver for the town was agricultural shipping, just like Walnut Grove. Locke had the advantages of being on a line of the Southern Pacific RR and the construction by the railroad of a rail head and very large warehouse building. Special attention should be paid to a funky restaurant and bar called “Al the Wop’s,” which achieves consistently high praise for its food andatmosphere.
Pic 20: A museum in Locke.
Pic 21: The famous Al the Wop's
If nothing else, the Delta invites a slow meander along the roads that follow the tops of the levees. From there you can see down into the farmland and ranches and over the rivers and streams that lace the area. It provides an opportunity to study the only large inverted delta in the world and the beautiful scenery one can see on all sides-scenery and resources we all want to enjoy and preserve for future generations.
Pic 22: Roads along the tops of the levees wandering through the Delta.