The San Lorenzo River was transformed by early logging in the Santa Cruz Mountains

The San Lorenzo River in Henry Kawell Redwoods State Park runs through deep waterways well below the historic floodplain. Credit: Will Chapman

The entrance road to Henry Cawell Redwoods State Park crosses the San Lorenzo River and winds through wide, flat meadows. Topographer Noah Finnegan, who studies rivers, does not look completely right about this landscape.

“The river is just below the floodplain. The meadow at the entrance is a historic floodplain and is the most natural. River Flood from them channel Finnegan, a professor of earth and planetary sciences at the University of California, Santa Cruz, said:

It was clear to Finnegan and his students that at some point in the past, the San Lorenzo River had flowed into the waterway and abandoned the floodplain. So they tried to find out when and why it happened.Their findings, published in December in the journal Earth Surface Processes and Landforms, link this. Dramatic change In the river’s actions against logging practices in the Santa Cruz Mountains in the 1800s.

Researchers have determined that the river is about 2-4 meters (6.5-13 feet) below where it would be in equilibrium with the floodplain. The river has risen over the embankment only twice (February 1998 and February 2017) since 1987, according to the records of the U.S. Geological Survey’s river flow meter in Felton, both of which have this event. It was not accompanied by widespread flooding in the area.

The San Lorenzo River was transformed by early logging in the Santa Cruz Mountains

Researchers stand on the left side of the abandoned floodplain and on the right side of the San Lorenzo River. The horizontal area in the center of the photo may be a new floodplain being formed.Credits: University of California Santa Cruz

To understand when channel deepening occurred, researchers used radiocarbon dating.

“We were able to find charcoal from the floodplain sediments that would tell us when the material was deposited,” said William Chapman, a graduate student at UCSC who led the study. “The latest dates from radiocarbon dating were 200 to 400 years ago, so the river led to floodplains from the 1600s to perhaps the 1800s.”

Clear-cutting spread in the Santa Cruz Mountains from the 1800s to the early 1900s and brought about dramatic changes in the landscape. One of the common effects of felling is to increase the rate of erosion of the basin by removing the vegetation that holds the soil in place, clogging rivers and streams with excess sediment and silt. But this will raise the riverbed of the river rather than cutting deeper waterways.

However, many common practices adopted early in logging lead to deeper channels. Since rivers are often used to transport logs, waterways are washed by logs dragged along the bottom, and riverbeds remove debris and logs that may have slowed the flow of water. Loggers sometimes built “splash dams” to control water and released them all at once to move logs downstream.

The San Lorenzo River was transformed by early logging in the Santa Cruz Mountains

When the San Lorenzo River cut a deep waterway, many such “hanging tributaries” remained. There, the water drops a few feet down to the main channel.Credits: University of California Santa Cruz

An important element of a river’s behavior is the balance between its “transport capacity” (the amount of sediment that the river can carry at high flow rates) and the supply of sediment that enters the river from the surrounding landscape. When the sediment supply exceeds the transport capacity, the river deposits sediment and raises its bed. However, when the transport capacity exceeds the sediment supply, the river begins to flow into the bed and deepens the channel in a process called “incision”.

“I don’t know what logging took place in the Felton area, but anything that simplifies the canal, such as slipping through the canal or pulling out logs, allows the river to transport sediment more efficiently. It leads to that, “says Finnegan. “When moving by changing the oil pressure Sediment More efficiently, the river begins to dig down and dig up its own bed. “

A 2015 survey of another basin in the Santa Cruz Mountains north of Santa Cruz reported similar results. The deep cuts in Pescadero and Butano Creek were due to well-documented historic logging practices such as log skidding in waterways and splash dams.

Floodplain abandonment has ecological consequences for species that use floodplain habitats such as salmon and rainbow trout. But now that the river has cut such a deep channel, there is not much that can be done to return it to the old floodplain. The river now seems to be stable, and in some places it appears to form a new floodplain underneath the old floodplain.

The San Lorenzo River was transformed by early logging in the Santa Cruz Mountains

Researchers have taken sediment cores from the historic floodplain of the San Lorenzo River for radiocarbon dating.Credits: University of California Santa Cruz

“The river is where I want to be now,” Chapman said. “The river cuts are irreversible, but at their lower levels they erode old embankments and create new equilibrium floodplains that can provide habitat. Floodplain Race. “

Finnegan said he learned a lot about the history of the San Lorenzo Valley while studying the topography of the river. He learned, for example, the waterways built to transport logs and the large gunpowder factories operating on the banks of rivers in the Paradise Park area.

“What’s interesting to me is the intersection of history, archeology and earth science,” he said. “We tend to take for granted what things look like in nature, but the enormous heritage effects of land-use decisions made decades and centuries ago still affect rivers today. It is influential. The forest has been restored, so it is difficult to understand how this area was extensively recorded and had its impact. ”

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For more information:
William AL Chapman et al, the abandonment of river floodplains and the deepening of waterways coincide with the start of clear-cutting in the Sequoia forests along the California coast. Surface processes and terrain (2021). DOI: 10.1002 / esp.5299

Quote: The San Lorenzo River was acquired from on February 14, 2022 in the Santa Cruz Mountains (February 14, 2022). Changed due to early felling in (Sun)

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The San Lorenzo River was transformed by early logging in the Santa Cruz Mountains

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