Erosion in the valley is a both a prehistoric and modern phenomenon. The tops of the Santa Cruz mountains have been losing elevation for millennia as the rain, wind and frost work to break down their structure to create boulders, rocks, peddles, sand and soil. But the hill slopes don’t look like they did 10,000 years ago. The current location of the Creek is not where it was when the Ohlone fished and gathered along its banks. Changes in the watershed have changed the balance and created some unhealthy conditions.
In the Age of Agriculture, it was found more efficident to channelize the creek and move it to the south side of the valley to maximaze arable land. And as the The rain, fog then works to transport the fragmented earth and The current location of San Pedro Creek does is far from., When agricultir, the desire to gte channelize dthe creek to the North Side . Becasue of this the creek is just begenning its. Is this a loosing? Will the San Pedro Creek not be until it finds its? No humans have altered the landscape and will not . But there will be some bums along the way . It will never be back to where it was in the Holcene or while the Ohlone banks.
Healthy Creek Course
High in the watershed, an abundance of native plants starts by slowing the velocity of the water coming from the clouds. The rain drops splash on the leaves of Toyon, Coyote Bush and Wax Mrtyle, slowing their velocity and eliminating their eroding energy. From the leaves, the water drips on to the dead leaf detritus, further slowing the water and hampering its velocity. Here is is absorbed into the soil, filtered clean as it makes its way into the aquifer and makes its way subsurface to the creek.
At the valley floor, the creek has room to meander as the Red Alder and Willow grows in the rich soils, changing the course of the Creek with their root structures. As the 50-year life cycle of the Alders turn, they die and rot out as new trees start their cycle on the opposite bank.
Healthy Erosion - The Sediment Cycle
The transport of boulders, rocks, sand and soil down the Creek has resulted in 1) A beautiful sand and boulder beach known as Linda Mar, 2) rich soil for generations of agriculture and grazing in the valley and 3) perfect sized, shaped and graded gravel for the Steelhead to create redds (nests) to spawn and hatch their young. The transport of the earth down the valley is of paramount importance and if prevented orstopped can have seriously deleterious consequences for the watershed.
In Stream Erosion - Incised Banks
Since San Pedro Creek is not in its prehistoric location, it will essentially want to work the earth until it finds an equilibrium gradient. There are parts (Flood Plain below Peralta, San Pedro Valley Park above Oddsted Blvd) where this seems to have happened but most of the natural channel of the Creek is finding its bottom. This is manifested in the presence of high and steep banks - know as incised banks.
With exacerbated high winter flows, without ample barriers in the stream bed to reduce velocity, and without sufficient stream side vegetation holding the banks in place - these steep banks will continue to get higher and steeper as the creek cuts deeper into the valley floor.
In Stream Erosion - Collapsing Banks
The next geological step after Incised Banks is Bank Collapse or landslide.
Its is unknown what the hills above Pacifica looked like before human inhabitants arrived. Was it forested with redwoods as the hills of Santa Cruz and Sonoma County are now? Were there other conifers and hardwoods? Or has it always been the hardscrabble Coastal Scrub we have now or is this just a result of human intervention?
It can be said with certainty that when the Spanish Ranchers of the 18th Century raised cattle in the hills of Pacifica, the landscape changed drastically. Not only did the cattle consume all the young green shoots of Califonia Native Plants, the Ranchers also brought over early season grasses and plants from Europe and South America that although good for cattle feed, were also really good at displacing and out-competing the native fauna.
And the easiest fuel and most versatile building material of the 18th century? Wood. So if the hills were filled with any trees when the Spanish arrived, they were most likely quickly consumed without great chance of returning given the bovine residents and invasive plants.
Roots play a large role in holding the soil in place and with the reduction in native plant mass, a percentage of the topsoil probably was blow and washed away, furthering the challenging conditions for healthy plant life.
With the Gold Rush, the Eucalyptus tree became a hope for riches, a novelty plant and a fixture in Northern California Landscapes. It has proven vigorous its its ability to survive and eliminate any chances Native trees plants in its
As roads to Mount Montara, Half Moon Bay were built, gullies needed to be filled and cuts made across hillsides.
All of these items have decreased the Watershed’s ability to slow, capture and hold rain water. The water in our valley travels down the mountains faster than it ever has taking more of the topsoil in the process. This results in the red-brown hue of the Creek during and after storm events. The sands and silts eventually make it to the Pacific Ocean but can have disastrous effects on spawning Steelhead as the gravel beds needed for their reds become silted in.