The coordinates will bring you to a public viewing location within the Los Cerritos Wetlands, located between Seal Beach and Long Beach along the southern coast of California. The Los Cerritos Wetlands sit at a unique geological and hydrological transition zone along the lower reaches of the San Gabriel River near Seal Beach. Unlike much of the river’s upstream channel, which has been heavily engineered with concrete banks to control flooding and accelerate water flow, this area represents one of the last remnants where natural processes still influence how sediment and water interact. The presence of wetlands here—rather than elsewhere along the river—is largely due to a combination of low elevation, reduced gradient, tidal influence from the nearby Pacific Ocean, and the historical accumulation of fine-grained sediments that promote water retention.
About Coastal Wetlands
Coastal wetlands form where land, ocean, and freshwater systems meet. These environments are influenced by tides, river sediment, and biological activity. The Los Cerritos Wetlands are one of the few remaining coastal marsh systems in the greater Los Angeles region. Historically, wetlands stretched across thousands of acres along the Southern California coast, but urban development has reduced them dramatically.
Despite their reduction, wetlands like this still perform important ecological functions:
• Sediment trapping – Wetlands slow down water flow, allowing sand, silt, and clay to settle.
• Land building – Over time, trapped sediments slowly raise the marsh surface.
• Erosion control – Wetland plants stabilize soil and protect shorelines from wave action.
• Flood buffering – Marshes absorb excess water during storms or high tides.
Wetlands are constantly changing because of tidal processes, sediment movement, and biological activity.
Wetlands typically form under certain geologic conditions and have the following qualities:
1) flat land with little topographic and geomorphological differences along local distances
2) stream gradients (elevation change) near their terminus in the wetland shall be low so that the discharge is also low
3) silt and clay sized particles settle out, with sand sized particles only settling during seldom high discharge events
4) little to no difference between high and low tides
5) Flow is not stagnate and is continuous to prevent salt water infiltration
Tidal Influence
The wetlands here are strongly influenced by tides from the nearby Pacific Ocean. Twice daily, tidal waters flow into and out of the marsh through tidal channels. These channels carry sediment and nutrients into the wetland environment. As water slows within the marsh, fine-grained sediments settle out. Over time, layers of sediment accumulate and create the muddy soils typical of tidal wetlands.
If sediment accumulates faster than erosion removes it, the wetland surface slowly rises.
Marsh Vegetation and Sediment Stability
Plants such as pickleweed and cordgrass are adapted to salty, waterlogged soil. Their root systems trap sediment and help stabilize the marsh surface. These plants also reduce water velocity during tidal flows, which encourages additional sediment deposition. This process demonstrates a key principle in coastal geology: Biological activity can influence land formation.
The geology of the wetlands also plays a crucial role in natural filtration. The fine-grained sediments deposited here have low permeability, which slows the movement of water through the ground. As water spreads out across the wetland, it is forced to pass through layers of mud and organic material. This process acts as a natural filter, trapping suspended sediments, pollutants, and excess nutrients. Microbial activity within these saturated soils further breaks down contaminants, improving overall water quality before it eventually reaches the ocean.
Human Impact and Restoration
Over the past century, wetlands throughout Southern California were filled for ports, housing, and roads. The Los Cerritos Wetlands area has experienced significant modification due to oil extraction, infrastructure, and development. Today, restoration efforts aim to restore natural tidal flow and rebuild marsh habitat. These projects help reestablish the geological processes that originally formed the wetlands.
Why a Wetland here?
As the San Gabriel River flows downstream from the San Gabriel Mountains, it transports a wide range of sediments, from coarse gravels to fine silts and clays. In its upper and middle reaches, the river’s steeper gradient and higher energy allow it to carry larger particles efficiently. However, by the time it reaches the coastal plain near Seal Beach, the gradient decreases significantly. This drop in energy causes the river to lose its capacity to transport heavier materials, resulting in deposition. Over time, layers of fine sediments—especially silts and clays—have accumulated in this low-lying area, creating poorly drained soils that are ideal for wetland formation.
Another key factor is the interaction between riverine and marine processes. The Los Cerritos Wetlands lie within a zone influenced by tides, meaning that seawater periodically moves inland, slowing the outward flow of freshwater. This mixing reduces flow velocity even further, encouraging additional sediment deposition. These tidal cycles also help redistribute fine particles across the wetland surface, maintaining a relatively flat, marshy landscape. In contrast, stretches of the river farther inland lack this tidal influence, so water continues moving too quickly to allow similar wetland conditions to develop.
In essence, the Los Cerritos Wetlands exist where they do because the river’s energy diminishes, sediments settle, and tidal forces reshape the landscape. These combined geological processes create an environment where water lingers rather than rushes through, allowing wetlands to form and persist. This makes the area not only a valuable ecological habitat but also an important natural system for sediment capture and water purification along the lower San Gabriel River.
Works Cited
https://scwrp.org/projects/zedler-marsh-meadows-and-hellman-lowlands-restoration-at-los-cerritos-wetlands/
https://www.waterboards.ca.gov/losangeles/water_issues/programs/regional_program/Water_Quality_and_Watersheds/ws_sangabriel.shtml
https://www.pinterest.com/pin/477874210441524668/
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Los Cerritos Wetlands" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with the San Gabriel River and surrounding wetlands at the posted coordinates. This photo MUST be attached to your "found it" log.
3. Look at the shoreline on the Seal Beach side (this side) of the river. Is the shoreline made of sand, silt, or clay or a combination of these? Can you see an abrupt shoreline or is it more gradual and submerged underwater? This is best seen during low tide. Explain your observations.
4. Look at the shoreline on the Long Beach side (other side) of the river. Is the shoreline made of sand, silt, or clay or a combination of these? Can you see an abrupt shoreline or is it more gradual and submerged underwater? This is best seen during low tide. Explain your observations.
5. Is the water clear or murky? What may this suggest about the natural filtration that occurs in wetlands? Is filtration here adequate, or are particles getting making it down river?
6. How does the water velocity change as you move towards the center of the river? Does it slow down, or speed up? Explain why this might happen in a wetland environment.
7. Based on the list of geologic conditions that lead to the formation of wetlands (see cache description for the items listed as 1 through 5), name at least 2 that are easily visable.
8. Based on your answers to 7, explain why wetlands naturally formed here along the San Gabriel River and not elsewhere up river.