Modeling paleocoastlines

n Coastal Basics, we explain how wave energy and sediment transport shape the modern coastline. Now we look at how sea level rise and El Niño/La Niña phases of climate transformed the southern California coast during the Holocene.

Sea Level and Coastal Processes
The initial appearance of sand beaches is a crucial turning point in the recent evolution of the southern California coast. Sea level has changed dramatically during the last 20,000 years from a glacial lowstand to its present interglacial high level. Click on the sea level buttons below to view a sequence of coastline responses to sea level change.

Sand beaches usually form at high seastands, and the last interglacial highstand 125,000 years ago brought wide, perennial sand beaches to the southern California coast.

During the last glacial period around 20,000 years ago, sea level stood about 120 m below present and streams cut deep valleys as they crossed the exposed shelf. The power of streams to transport erosion materials was increased by the steeper gradient between the mountainous watersheds and the lowered sea level.

Coarse materials like cobbles and boulders were washed to the coast, and the deltas formed by these materials were gradually dispersed by waves and currents to build cobble beaches along the coastline.

As the glaciers melted and sea level rose rapidly, rocky coasts were submerged and estuaries expanded up the stream valleys. Estuarine habitats of the Early Holocene (11-8,000 years ago) were rich environments teeming with fish, birds, and shellfish. Sediments were now deposited in the heads of the embayments, leaving the open coast with cobbles and rocky intertidal habitats.

When sea level rise slowed around 8-6,000 years ago, waves began cutting the present shore platform, and sediment brought to the coast during floods formed the first sand beaches. The estuaries became tidally flushed lagoons with sand and mud flats. Rocky intertidal and shallow reef habitats were buried by sand as the littoral cells' transport mechanisms were established.


ENSO Forcing of Wave Climate and Sediment Flux

In Climate Change/The Present, we describe how El Niño influences both wave climate and sediment flux. Southern California's coast is particularly sensitive to ENSO state. During El Niño winters, anomalously high rainfall and wave energy cause floods and coastal erosion. La Niña-type climate brings milder, drier winters particularly in the southernmost region of the coast. Wet, El Niño-dominated climate can last for several decades, followed by 20-30 years of dry, La Niña type climate. Wave climate changes dramatically between El Niño and La Niña-type winters.

Sediment yield to the coast is also strongly affected by these wet and dry climate periods. Wet periods with many El Niño winters bring twice the amount of sediment to the coast of the Santa Maria region as is supplied during the dry periods. The Santa Barbara coast receives 5 times more sediment in wet periods because of its highly erodible, mountainous terrain. And on the San Diego coast, the wet period is almost the only time that sediment is supplied to the coast.

 Sediment yield in dry and wet decadal periods for southern California coastal provinces
(data from Inman and Jenkins 1999).

How did Holocene climate affect the coast?
Climate proxies tell us that El Niño events were extreme but infrequent in the period 12-8,000 years ago. A long paleoclimate record from an alpine lake in Ecuador shows a systematic change in El Niño activity during the Holocene. Between 8,000 and about 5,000 years ago, winters were dry. Starting around 5,000 years ago, El Niño floods became more frequent and the intervals about 3,000 and after 2,000 years ago were particularly stormy.

The shift in wave climate around 5,000 years ago would have rapidly eroded beaches on exposed east/west trending coastlines and on beaches that had been sheltered from the earlier La Niña wave climate by headlands and the Channel Islands. Sediment flux to the coast would have increased most markedly in the southern cells due to increased precipitation after the long, dry period 8-5,000 years ago.

Explore the Holocene evolution of the Santa Maria, Santa Barbara, Oceanside, and Silver Strand littoral cells in the next four sections.