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Place 02 · Monterey Bay · Chapter 6 of 7
The Canyon That Eats the Beach
Every beach in Monterey Bay is slowly walking sideways toward a hole the size of the Grand Canyon.
About 15 minutes · 1 video · 1 3D map · 2 quick activities
Start here
A beach is a river of sand
Stand on a beach in Santa Cruz and the sand under your feet is moving, a few steps down the coast with every wave. It came out of the mountains by river, it is walking along the shore, and it is headed for the top of Monterey Canyon near Moss Landing. Once it gets there, it slides into the deep sea and never comes back.
Watch this
See it move
Scientific American — This map of a Grand-Canyon-sized chasm off California took 15 years to create. A canyon this big, this close to shore, and almost nobody on the beach knows it is there.
While you watch, look for
- How big the canyon is, compared with something you know on land
- Where the canyon starts, and how close that is to the beach
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The conveyor
Waves hit at an angle
Waves almost never arrive head-on. Each one pushes sand up the beach at a slant, then gravity pulls it straight back down, so every grain zigzags along the shore. Add up millions of waves and you get longshore drift: a river of sand running parallel to the beach.
The budget
Rivers in, canyon out
A littoral cell is a stretch of coast with its own sand account. In Monterey Bay the deposits come from rivers like the San Lorenzo, Pajaro and Salinas and from crumbling bluffs, and the big withdrawal is the canyon head at Moss Landing, where sand from both ends of the bay piles up and then slumps into the deep.
The nursery
Where fresh meets salt
Right beside the canyon head, Elkhorn Slough winds nearly seven miles inland: an estuary, where river water mixes with the sea. Its salt marsh is the second largest in California, and it is crowded with young fish, leopard sharks, bat rays, shorebirds and sea otters cracking crabs in the channels.
Photos: Griffin Wooldridge, Denitsa Kireva, Jonathan David / Pexels
Play with it
Fly down Monterey Canyon
This is a 3D map of the real seafloor under Monterey Bay, built from sonar depth measurements. Drag to spin it and scroll to zoom, then follow the canyon from its head near the shore out into the deep.
3D model: Bathymetry of Monterey Bay Canyon by DeepSeaBEE, on Sketchfab (CC BY 4.0).
Two questions to answer while you are in there
- Where does the canyon start: far offshore, or almost touching the beach?
- Sand walking along the shore reaches the canyon head. Which way does it go next, and why can it never walk back?
Try it yourself
Check yourself
Try it
Sand In, Sand Out
Click every word that names a source of new sand for the beach, and leave the sand thieves alone.
Try it · interactive
Sand Budget
Run Monterey Bay’s sand budget: let rivers deliver sand, let the longshore current carry it, let the canyon swallow it, then add a dam or a seawall and watch the beach get narrower.
Open the Sand Budget in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Longshore drift
Sand zigzagging along the beach with waves
Littoral cell
A stretch of coast with its own sand budget
Submarine canyon
A deep valley cut into the continental edge
Estuary
Where river water mixes with seawater
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Canyon That Eats the Beach
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 13.2 — Longshore Transport — LibreTexts · 10 minutes
- Oceanography 101 (MiraCosta) 12.12 — Coastal Littoral Cells — LibreTexts · California examples · 10 minutes
- Webb 13.6 — Estuaries — LibreTexts · 10 minutes
Photo credits (Pexels): RDNE Stock project, Jonathan Borba, Griffin Wooldridge, Denitsa Kireva, Jonathan David.