Human Effects of Logging on the San Lorenzo River

Up next in our Science Committee’s Naturalist Segment is an over view of Logging on the San Lorenzo River written by Sarah Faraloa.

The San Lorenzo River has a rich and complex history. For the early inhabitants of Santa Cruz, the San Lorenzo River was the central water source and is the reason downtown Santa Cruz is built where it is today (Figure 1). As a mountain biker, you may think you only interact with the river on your way to and from shredding the trails. Perhaps you notice the San Lorenzo when you are cruising along the river path from the Boardwalk trestle heading into the mountains, or riding high above its running waters as you rumble along the railway tracks in Henry Cowell. I became more intrigued by the San Lorenzo when I learned about a sneaky river secret that I ride over a few times a week, high above where the river runs today. You are probably familiar with it too - the rounded river rock cobbles that cover the single track right before you enter the trees on the Emma McCrary trail (Figure 2).

Figure 1. The San Lorenzo River

I was interested in learning more, so I contacted UC Santa Cruz professor Noah Finnegan, who is a fluvial geomorphologist (a special style of human who is obsessed with how the landscape changes as a result of the mysterious ways of rivers) and inquired about these peculiarly placed river cobbles. Noah believes that these cobbles are old San Lorenzo River gravels that were deposited long ago when the river was at a higher elevation. Incidentally, there are similar cobble outcrops like this, but even higher above river level, in Henry Cowell State Park on the Eagle Trail. Exactly how and why these gravels are deposited in these locations is still unknown.

Figure 2. River Cobbles along the Emma McCrary Trail

These river gravels show that there are still mysteries about this river that have yet to be uncovered. Another intriguing San Lorenzo mystery was revealed to me after talking with Will Chapman, who researches fluvial geomorphology with Noah Finnegan at UCSC. The question Will is attempting to answer is how logging-related activities have affected the San Lorenzo River, and specifically, why the present-day San Lorenzo river is about 2 meters greater than we expect to see, from the depth of the riverbed channel to the floodplain, based on hydrologic and geologic modeling.

Clear-cutting and logging of redwoods in the Santa Cruz mountains reached its peak in the late 1800s, where there were over 30 logging sites built up along the San Lorenzo River and its tributaries. Clear-cutting a redwood forest has a few significant effects on the landscape. First, the removal of trees causes water to flow faster over the land, resulting in much flashier floods. Second, redwoods are very efficient at intercepting rainfall, so after their removal, water that was previously caught by the trees and evaporated back into the atmosphere is instead directed into the river. Third, and perhaps most importantly, loggers employed a practice called splash damming, in which they built temporary dams, filled the pool behind the dam with harvested logs, then blew out the dam to transport the logs downstream (Figure 3). Logjams, which often cause sediment accumulation on river bottoms, were also removed in this process. Splash damming and clear cutting both resulted in more energetic and abrasive flows of sediment-laden water in the river, which, over a long period of time, caused the river to erode its own bed and lower substantially. This is called river incision, and is important from both geological and ecological perspectives. Based on the present-day balance of hydrology and sediment supply, the banks of the San Lorenzo River should be 2 meters higher than what we actually observe. This is significant; the channel is so deep that the river floods over its banks only once every decade or so. One consequence of this is the loss of critical riverbank habitat for a variety of now endangered species, such as red-legged frogs and rainbow trout. From 1842 to 1940 these logging techniques were used, and the San Lorenzo incised down into the rocks below, severing the connection between the river and its floodplain.

Figure 3. Loma Prieta Mill, Santa Cruz Museum of Art and History

To constrain the timing of the 2 meters of river incision, scientists took a sediment core along the bank of the San Lorenzo River, and analyzed the bits of charcoal that were found in the floodplain sediments. The charcoal-containing floodplain sediments shed light upon a few interesting questions:

1) The dates of charcoal tell us when the river was still flooding and depositing sediment onto its floodplain,

2) There are no dates more recent than around 1800, (suggesting the river incised some time after that), and we know logging began in 1842 and ramped up in the 1870’s.

This suggests that the San Lorenzo regularly overtopped its banks until logging began, and changes to hydrology and logging practices caused the river to cut downward and abandon its floodplain. The effects of clear cutting a forest combined with logging practices caused the San Lorenzo River to shift away from its natural state and evolve into a human-influenced river, which we see evidence of today as a lower river than expected.

There you have it! It is quite satisfying (and rare, I might add), that a nice neat answer can be discovered in science, in the short time frame of a Master’s degree (2 years). There are still many exciting mysteries to be uncovered about the San Lorenzo River, but today you can be either satisfied or unsettled knowing that logging is the likely culprit of the 2 meters of river incision we observe today. Also, the gravel that you pedal over on Emma trail is not there by chance, but through the epic history of the legendary San Lorenzo River. Who would have thought that humans in the 1800s could have such a significant influence on the shape of the river? It makes you wonder what our future successors will think about our influence in 200 years.. (haha ;) )

Figure 4. Map of logging along tributaries of the San Lorenzo

Sarah is a member of SCMTS’ Science Committee. She's from Kauai and graduated from UCSC in Earth Sciences. Sarah now works in Natural Resource Planning in the Monterey Bay Region. In her free time she loves to surf and mountain bike with friends, cook feasts, and read/learn/discuss science and the nature of things!