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

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The entrance road to Henry Cowell Redwoods State Park crosses the San Lorenzo River and winds through a wide, flat prairie. To Noah Finnegan, a geomorphologist who studies rivers, something about this landscape doesn’t seem quite right.

“The river is really deep below the floodplain. This meadow at the entrance is the historic floodplain, and most natural rivers overflow their channel every one to two years, but the San Lorenzo never floods to Henry Cowell,” said Finnegan, professor of science at Earth and planets at UC Santa Cruz. .

It was clear to Finnegan and his students that at some point in the past the San Lorenzo River cut itself in its channel and abandoned the floodplain. So they set out to find out when it happened and why. Their findings, published in December in the journal Earth surface processes and landformsconnect this dramatic change in river behavior to logging practices in the Santa Cruz Mountains in the 19th century.

The researchers determined that the river is about 2–4 meters (6.5–13 ft) below what it would be if it were in equilibrium with the floodplain. Records from the US Geological Survey’s Felton flow gauge show the river has only crossed its banks twice since 1987 – in February 1998 and February 2017 – and neither event involved widespread flooding in this area.

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

“We were able to find charcoal in the floodplain sediments that could tell us when the material was deposited,” said William Chapman, a graduate student at UCSC who led the study. “The last dates we got from radiocarbon dating were between 200 and 400 years ago, so the river was still connected to its floodplain in the 1600s and probably the 1800s.”

Clearcutting was prevalent in the Santa Cruz Mountains in the 1800s and early 1900s, bringing dramatic changes to the landscape. One of the common effects of logging is to increase erosion rates in the watershed by removing vegetation that holds the soil in place, causing rivers and streams to become clogged with excess sediment. and silt. But that would raise the river bed rather than digging a deeper channel.

However, many common practices employed in early logging would lead to a deeper channel. Rivers were often used to transport logs, so the channel was carved out by logs dragged along the bottom and the streambed was cleared of any debris or ice jams that might have slowed the flow of the river. ‘water. Loggers sometimes built “splash dams” to hold back water, then let it flow all at once to move logs downstream.

A key factor in the behavior of a river is the balance between its ‘carrying capacity’ – the amount of sediment the river can transport during high flows – and the supply of sediment from the surrounding landscape. If the supply of sediment exceeds the carrying capacity, the river will deposit sediment and raise its bed. But if the carrying capacity exceeds the supply of sediment, the river will begin to deepen its bed, deepening the channel in a process called “cutting”.

“We don’t know for sure what logging practices were used in the Felton area, but anything that simplifies the channel, like dragging logs through the channel or removing traffic jams, causes the river to carry sediment more efficiently,” Finnegan said. “When you modify the hydraulics to move sediment more efficiently, the river will start to deepen, mining its own bed.”

A 2015 study of another watershed in the Santa Cruz Mountains north of Santa Cruz reported similar results. The deep incision of Pescadero and Butano creeks has been attributed to well-documented historical logging practices, including the skidding of logs in canals and the construction of splash dams.

Floodplain abandonment has ecological consequences for species that use floodplain habitats, including salmon and rainbow trout. Now that the river has carved out such a deep channel, there is little that can be done to restore it to the former floodplain. The river now appears to be stable, and in some places it appears to form a new floodplain beneath the old one.

“The river is where it wants to be now,” Chapman said. “You can’t reverse the river incision, but at this lower level it will erode old banks to create a new equilibrium floodplain that could provide habitat for floodplain species.”

Finnegan said he learned a lot about the history of the San Lorenzo Valley by studying the geomorphology of the river. He learned, for example, of the existence of a canal built to transport logs and of a large powder mill that operated on the banks of the river in the Paradise Park area, the traces of which can still be seen.

“What interests me is the intersection of history, archeology and earth science,” he said. “We tend to take for granted how things look in the natural world, but there are huge legacy effects of land use decisions made decades and centuries ago that still affect rivers today. today. The forests have grown back, so it is difficult to appreciate how much this area has been exploited and the impacts this has had.

In addition to Chapman and Finnegan, the paper’s co-authors include Allison Pfeiffer of Western Washington University and SeanPaul La Selle of the US Geological Survey. This work was funded by UC Santa Cruz and the UCSC Department of Earth and Planetary Sciences.

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