Large Woody Debris in San Geronimo Creek, 2016-2017

This is the story of wood in San Geronimo Creek pools immediately upstream of the Meadow Way bridge during the 2016-2017 winter season. When it arrived, and when it, sadly, departed.

Looking north along the bridge railing during the biggest flood
in a decade (December 2012) on the left and September 2016 (right).

September 18, 2016: Before the floods.
Looking upstream.

The point bar immediately upstream of the bridge tends to grow alder saplings during low-intensity rain years and be scoured in years with higher intensity rainfall and runoff. In September 2016, the alders were looking bigger than at anytime since 2011. I had high hopes they might grow into trees big enough to withstand scouring winter floods.

December 15, 2016: Looking upstream during the biggest flood of the 2016-2017 winter.

December 20, 2016: Looking upstream after the flood--still a few
saplings left on the bar on the right.

January 4, 2017

January 5, 2017

January 10, 2017. In the middle-right of the photo, one of the trees
that Marin County removed in May arrived.

January 11, 2017. The tree at the center-top is now visible in
roughly the configuration that it stayed the rest of the winter. This was
far enough upstream of the bridge, and a small enough tree, that it
seemed hopeful the county would leave it alone since it provided
much-needed habitat for threatened Coho Salmon.

January 15, 2017. The tree and its rootwad
are more visible as the flood recedes. 

January 31, 2017. At least one flood during the previous two weeks
scoured the ivy off the tree and began creating a pool around the rootwad.

February 7, 2017. Another large flood, but the tree didn't budge.
This was the set of storms that triggered many mudslides in the area.

March 4, 2017. The property owner cut off the top of the tree.

March 7, 2017. Another view, with a Japanese Knotweed patch
visible at the center of the photo. Japanese Knotweed is a noxious
weed that is easily spread and difficult to eradicate. fact sheet /
property values article / newsweek article

April 8, 2017. Spring has sprung! And at least one of the alder saplings
on the bar made it through the winter!

April 23, 2017. A large alder fell on the bridge and
into the creek, cracking into at least two pieces and
breaking the bridge railing 

This is amazing fish habitat: cover and shade. Very large wood
also slows down water, and reduced velocity from the added roughness
can limit erosion in an incised system like San Geronimo Creek.
With California's Mediterranean climate, there is absolutely no
reason to remove a log like this prior to fall, which would allow the
habitat to benefit fish all summer. Even after fall, the break in the
middle of the log would tend to limit any threat of a debris jam forming
against the bridge during a winter flood.

May 18, 2017. Marin County removed BOTH logs--even the one
that persisted well-upstream of the bridge through the big storms
since January and had created a deep fish-nurturing pool around
its rootwad.

And to add insult to injury, the single-remaining alder sapling on
the bar appears to have been cut as well.

Meanwhile, downstream, there are very expensive projects to introduce large wood habitat structures into Lagunitas Creek. The upstream tree that was removed was stable enough during the February storms that it could have been cabled into place had the road department been taking a holistic approach. If left, it could have collected debris prior to reaching the bridge, thus making the bridge less likely to collect debris. If you imagine much more wood and trees left in the creek, the additional channel complexity and roughness would tend to catch anything else falling in, making it less likely that a tree falling in and floating downstream would get very far. By removing all the wood, it makes it more likely that a tree falling in will float downstream to a bridge and create a jam.

The senseless removal of large wood from San Geronimo Creek is probably a large part of the reason why Coho Salmon are not doing well in this creek. At a minimum the wood should have been left in until fall so that it could have provided shade and cover all summer. Cutting trees and removing wood from the incised floodway promotes faster velocity floodwater and further incision and erosion. Leaving wood in the creek also protects the banks from erosion directly, like a wood-paneled channel. As trees are removed over time, San Geronimo Creek is slowly being transformed from a creek into a ever-widening and deepening flood control channel. Each time wood and vegetation are removed, it loses a little more habitat value for the creatures that live only here, and becomes a little less special, and becomes a little more like everywhere else.

"One of the penalties of an ecological education is that one lives alone in a world of wounds. Much of the damage inflicted on land is quite invisible to laymen. An ecologist must either harden his shell and make believe that the consequences of science are none of his business, or he must be the doctor who sees the marks of death in a community that believes itself well and does not want to be told otherwise." --Aldo Leopold