Rise and Fall of San Diego – Part 2 of 2

[ Music ] >> Hey. I'm up here, on top of the San Diego National Bank building. From here we can see San Diego Bay with its marine traffic and it really is a superb bay for its commerce, its recreation and, of course, its beauty. But 120,000 years ago, had this building been here, it would have looked like this. [ Sound of Rushing Water ] So why would this building be under water? For more than 2 1/2 million years, the earth has been locked in an ice age. In fact, sheets of ice up to 2 miles thick once covered 25% of North America. It was very much like Antarctica and Greenland are today. So, where did the ice come from and where did it go? >> Well, the earth constantly changes. In geological time, continents drift and when the continents drift, they may divert warm, equatorial ocean water to the polar latitudes.

Warm seawater evaporates into the atmosphere. That moisture is carried aloft and crystallizes as snow in northern latitudes. It begins an ice- building cycle. When earth's orbit and tilt cause less solar energy to reach us, much of the winter snow does not melt during the summer. It simply becomes deeper and heavier. So, the lower layers are compressed into solid ice. And because ice is weak, it deforms and then flows farther across continents. It is not returned to the ocean. Simply stated, the evaporated sea water is stored on land as ice, which means the sea level drops. And as earth's orbit and tilt continue to change, a warming cycle returns, just the opposite occurs. The ice melts and the sea level rises. [Music] >> And this cycle of rise and fall, rise and fall, happens many times throughout geologic history. In fact, just 11,000 years ago, the global sea level was 400 feet lower. What did San Diego look like then? >> There was no San Diego Bay, no Mission Bay, no Silver Strand.

They have all formed in the last 9,000 years. The latest retreat of the continental ice sheets occurred rapidly, in less than 5,000 years. As sea level rose, the San Diego shoreline looked different: one huge bay with three islands. >> Today, Mission Beach is a unique and lively beach community enjoyed by residents and tourists alike. But 9,000 years ago, it simply was not here. Now, Mission Beach was not created by uplift and the tremendous forces of the Rose Canyon Fault, just 2 1/2 miles to the west, did not create Mission Beach. Mission Beach was created one wave at a time, day by day, year by year, century by century. >> Waves arriving from the North Pacific Ocean hit most west coast beaches at an angle. Because they hit at an angle and because most of the waves came from the northwest, they pushed tons of sand southward along the coast. As the sand collected, it built southward into what we now call Mission Beach. >> But other natural forces began to factor into the equation and all three of these islands would be greatly affected. These surfers come out here for one reason, to ride the waves.

Hard core surfers study and know how global weather patterns affect their particular part of surfing paradise. In this instance, the forces they are riding have created a significant part of San Diego today. >> Over the millennia, as each flood season came and went, the ancient Tijuana River dumped millions of tons of sand and mud into its delta, which was built out into the ocean. During the late summer, hurricanes in the southern hemisphere, some as far away as New Zealand, generated enormous waves. And as they came, they picked up the sand from the river's mouth and began to redistribute it. They began to build a barrier. And as the barrier became larger and longer, over 6 miles in length, it connected with two islands, Coronado and North, and they became tied to the mainland. Today, we call this 6 mile barrier, the Silver Strand, and though we refer to Coronado Island and North Island, they are now, really, part of a peninsula. So, while surfers enjoy the late summer waves on the ocean side of the beach, we realize because of Silver Strand in the south and Mission Beach in the north, the coastline is no longer open to the ocean.

It is now, for the first time, a bay. But, how did this mega bay get divided into two with San Diego Bay to the south and Mission Bay to the north? And what about the third, and largest, island? Well, it was positioned just off the coast, opposite the mouth of the ancient San Diego River. Like the ancient Tijuana River, as each flood season came and went, the San Diego River dumped enormous amounts of sand and mud into its ever- expanding delta and as the delta grew, it reached farther out into the bay. A few thousand years ago, it met the island and connected it to the mainland and it split the mega bay into two smaller bays. To the north is Mission Bay and in the south is San Diego Bay. >> Today, the island is called Point Loma and the land that grew outward as the San Diego River delta is called the Midway District, which is where I am right now. Now, the ground may appear firm enough, but in actuality, it is made of loose sand. The river has simply dumped it into the bay. By geological description, it is a weak, water-saturated material, but it has become the foundation for some of San Diego's most widely used structures.

In fact, all of the sewer pipes from San Diego cross this unstable sediment on their way to the sewage treatment plant on Point Loma. When we have a major earthquake on the Rose Canyon Fault, this property, the Midway District, will liquefy. Our human built structures will fail, no operating Lindberg Field, no sewage treatment, of course, badly damaged roads and all of the other problems that accompany a major fault movement. But another reality is, even when we have a major earthquake, the worst we can imagine, it will still be a simply chiropractic adjustment in geologic terms and the process never stops. The earth is always shedding its skin. >> So, let's speed up the last million years and see what happened. Regional uplift caused the retreating sea to cut Linda Vista and Otay Mesas as glacial advances and retreats lowered and raised the sea level many times. Movements within the Rose Canyon Fault warped and shifted the land, creating Mount Soledad, Mission Bay and the three islands of Coronado, North and Loma.

Ocean waves from the north pushed sands south to form Mission Beach, just as summer waves from the south washed Tijuana River delta sand to the north to form the Silver Strand that captured Coronado and North Islands. Meanwhile, the San Diego River delta built outward and attached the big island to the mainland to create Point Loma. >> So, here we are in the now. And knowing what we know about the past makes it fun to think about what the next million years might bring. So, let's take an educated guess. >> If present trends continue, Linda Vista and Otay Mesas will continue to rise. The Rose Canyon Fault will squeeze Mount Soledad even higher and drop Mission Bay even lower. And remember, the continental ice sheets both grow and shrink. When they shrink and retreat, melting ice on Greenland and Antarctica will cause the sea level to rise 200 feet, forming beach front property in Mission Valley and San Ysidro. Conversely, when glaciers grow and advance, the sea level will drop 400 feet lower than now, several times, emptying San Diego and Mission Bays, leaving the present coastal communities high and dry and making the Coronado Islands part of the mainland.

>> It is certainly interesting to speculate, but the truth is, every time we think we have this old world figured out, she manages to surprise us. And anyway, that's a million years from now. We might as well go surfing. [ Music ].