Rainstorms Ahead, Sutro Forest Can Respond If We Let It

The National Oceanic and Atmospheric Administration predicts that El Nino could arrive within a couple of weeks. (Here’s a report from the California Business Journal.) It looks like we’re going to have a very wet winter. We think it’s urgent to stop all vegetation removal in the forest now as a safety measure. One of the risks of cutting down trees and removing vegetation – as has been happening in Sutro Forest – is it reduces the ability of the forest to respond to the rain and wind that winter brings.


Most of Sutro Forest is on steep slopes, some of which can become unstable. Yet we have had no recent landslides in the forest. Why? Largely because the intergrafted roots of the dense tree growth acts like a living geo-textile and stabilizes the slope. This is crucially important, and we ask land managers for Sutro Forest (and for nearby Mount Davidson) to focus on the real risk of destabilized slopes. The risk is not just academic, as is evident below.

Landslide under blue tarp. South Ridge of Sutro Forest at top left.

Landslide under blue tarp. South Ridge at top left.

Forest Knolls is a neighborhood of homes just south of the forest. In the picture here, the forest is actually visible in the top left corner. The blue tarp in this picture was where a slope started sliding after vegetation removal. It remained there for a long time.

Inside the forest, an area at the top of Medical Center Way became unstable, after quite a lot of clearing on the East Ridge Trail just above it. The tarp in the picture below remained in place for over a year. Only recently has UCSF gone ahead with slope stabilization measures. But had the forest been left alone and no vegetation removed above this slope – they might never have been needed.

blue tarp above Medical Center WayHere’s a map showing the slope stability risk assessment (this came out of a UCSF publication for an earlier version of the plan to cut down thousands of trees.) The wiggly arrows show where there’s a slide risk, and the double arrows show where there’s evidence that slides actually occurred some time in the past. [Edited to correct reference regarding arrows.]

Pink areas and wiggly arrows show landslide risk; double line arrows show past landslides.

Pink areas and wiggly arrows show landslide risk; double line arrows show past landslides.

Nearby Twin Peaks, which is bare of trees, has had multiple rock slides. (In the picture below, Sutro Forest is in the foreground, and Twin Peaks is the brown pair of peaks behind Sutro Tower.)

sutro forest with approaching cloudsWhenever there’s a really wet winter, more rock slides happen. Since it isn’t surrounded by houses, this is more a nuisance than a real problem. SF MTA usually clears the rocks to the side within a day or two. However, if there were a house or car underneath – there would be real damage, even danger.

rockslide on Twin Peaks

Twin Peaks rockslide

As the tragedy of the community of Oso indicates, it’s possible to destabilize a slope by cutting down trees, and only feel the effects years later when weather conditions converge.


This forest is adapted to rain and wet conditions because it gets rain in winter and fog-moisture in summer. That’s why it’s a Cloud Forest. The force of the rain is broken by the layered canopy of eucalyptus, with a subcanopy of acacia and plum, and so it lessens the erosive impact.

The duff  and the dense understory vegetation protects the soil further, holding it together and allowing water only to percolate through, not bombard it. The whole forest floor is protected from erosion, except where the vegetation has been “cleared” for whatever purpose.

There’s some erosion on the trails – especially because many of them have been widened in the last few years, but on the whole it’s not nearly as bad as say, Twin Peaks.


Not only does the vegetation prevent erosion, it also regulates water run-off.  The vegetation traps the water as we described above. The Sutro Cloud Forest’s duff and understory holds water like a sponge. Then it gradually releases it over a period of days, not hours. This is important because a sudden flood of water through the system can overwhelm the sewer system of San Francisco. Everything we can do to slow the runoff helps.

The runoff from the mountain is a fraction of the runoff from, say, Twin Peaks, where the road runs like a river on rainy days. Removing undergrowth and duff and getting down to the soil increases the runoff and could also increase erosion.


This area is one of the windiest in the city.  With trees up to 250 feet tall, this forest is an important windbreak for surrounding neighborhoods.

Mount Sutro forest viewed from southeast (Twin Peaks)The trees also protect each other from the full force of the winter winds. This is true even for trees that are dying or even dead. This protective effect reduces the impact of winter wind-storms not just inside the forest, but also around it.

sutro deforested

If there was no forest there


Too often, when the talk goes to safety, people focus on fire hazard (much overblown owing to various myths).  Recently, the SF Fire Department exploded many of these myths.

The other risk factor frequently considered is “hazardous trees” – even though people are nearly twice as likely to die from a lightning strike as from a tree fall. And that’s a national figure that includes the East Coast which has more trees and more storms.

What is seldom talked about is slope stability, and the fact that the trees are helping to stabilize these slopes and prevent landslides. This is a real, long-term issue, where changes to the forest now could have adverse effects immediately – or up to seven years later.

So in preparation for the coming El Nino, the best thing to do would be – stop all removal of trees in whatever condition; and stop all removal of understory. This forest is adapted to rainstorms; its important not to destabilize it.


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One Response to Rainstorms Ahead, Sutro Forest Can Respond If We Let It

  1. apglk@comcast.net says:

    There is discrepancy in double/wiggly arrows: the phrase above the map states one thing, the phrase below – another: “The double arrows show where there’s a slide risk, and the wiggly arrows show where there’s evidence that slides actually occurred some time in the past.” “Pink areas and wiggly arrows show landslide risk; double line arrows show past landslides.”

    [Webmaster: Thanks for catching that! Corrected.]

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