So the weather is warmer, the sun is stronger, and San Geronimo Creek is low. This gets me thinking about walking up the creek again on a warm day. Last year I walked from our house up to the next bridge upstream. The first attempt ended at a neck-deep pool on a not-warm-enough day when I wasn't appropriately attired. Turning around and walking downstream ended at a debris-jam with a similarly-intimidating pool.
Now I look downstream and see that the debris jam appears to be gone. The set of storms and resulting peak flows this winter were not big enough to jam more debris, but big enough to clear out the debris already there. I want to go downstream to investigate.
The walk upstream finally came together last year when on a really warm day (the water is so cold!) I went up in just my shorts without water-sensitive items in my pockets. This is an important point, because it takes me an hour to empty my pockets of all the items that can be found in them. I brought the walking stick that my cousin Bill--a former CalTrout board member--gave me when he taught me how to flyfish. I was ready.
The neck-deep pool turned out to be not the deepest pool--just the first deep pool out of two or three between our house and the bridge. The bridge is about a quarter mile by road, so it is probably a half a mile by stream channel. People's backyards run right to the stream, or to the top of the steep incised banks above the stream, so going around the pool wasn't a comfortable option.
The trickiest thing about this pool was that there was a log across it at water level right in the deepest spot. So you had to go from almost-swimming to climbing over the log--in freezing cold water.
After passing the confluence of Sylvestris Creek (where there is some interesting old railroad debris from the defunct North Pacific Coast Railroad) and Creamery Creek (more about this creek later), there is a half-covered bridge overhead with a shed or garage built on it. Right after that there is a really sunny, shallow, algae-covered gravel reach. The bridge had a beehive in it and the bees were all going to this sunny algae-covered gravel that the flow was seeping through. I was a bit nervous about stepping on a bee and getting stung, but all was well going through this section.
So I mentioned Creamery Creek. The creek runs through a culvert and splashes down a steep concrete apron with boulders at the bottom, where it enters San Geronimo Creek. This culvert configuration would stop any fish from migrating upstream into Creamery Creek to spawn. It is probably expensive to fix because it is somewhat surprising that the county allows such a fish passage barrier to remain on such a major tributary.
 |
| Looking down on the Creamery Creek confluence with San Geronimo Creek, November 20, 2011 on a rainy day. |
When I walked by this confluence during winter storms and looked down at it, I could see the smaller Creamery Creek flow diving into San Geronimo Creek, creating a deep pool and eddy along the left side of the larger creek. This eddy caused a bar to form just downstream of the confluence, where the sediment from both creeks was deposited in the slow-moving water between the faster current of each creek. The bar was out in the middle of San Geronimo Creek after one of the big winter storms, separating the Creamery Creek flow on the left from the San Geronimo Creek flow on the right.
Yesterday I walked by and noticed that the bar had been pushed over to the left edge of the channel, and the Creamery Creek flow now would flow into San Geronimo Creek upstream and to the right of the bar. This got me thinking--which is where the title of this post comes in.
In rain-dominated systems like most of the coastal streams of California, you get several peak flows during the rainy winter season. The flows that exceed the threshold where bedload is mobilized will move the sediment that forms bar and pool features in the channel--and potentially will change their configuration. Smaller peak flows will set things up one way, but bigger flows will act differently (especially where vegetation or other roughness affects the flow) and deposit sediment and scour in different places.
The Creamery Creek - San Geronimo Creek confluence is one example where that bar moves in relation to the magnitude of the peak flow. But all along the channel this is happening, and whatever the last bedload-mobilizing peak flow of the season is would seem to have a disproportionate effect on the channel. This is because the configuration of the bars at the end of the rainy season is how they will be all summer until the next big peak flow in the fall. That means that vegetation can get established on those bars during the summer and that vegetation will in turn affect how future flows behave at that spot.
The largest peak flow of the season would have the largest effect on the channel, but could this "last large peak effect" be teased out?
How would you quantify this effect in a given stream? I hypothesize that several years of measurements would probably show this effect. Just tracking the bar at the Creamery-San Geronimo confluence between storms and year-to-year would be very interesting, not hard to do (simply a photograph would do), and perhaps it would be able to be correlated with the magnitude of the peak flows in between photos. Then comparing the largest peak and the last large peak would reveal when this is significant (presumably in drier years like 2012).
Why do this? Well, it would be really interesting. Maybe someone has done it already. I'm going to google it now to see if someone has. Results? Sediment movement is a rich (and complex) area of study and yes, they probably have.
So I just created the "Nerd Terrarium" category for this blog. So you other nerds out there can quickly find what you need. Thanks to the Daily Show's introduction of this term into the lexicon of my daily vocabulary.
No comments:
Post a Comment