Catching up on the February Storms

 It doesn't feel right to let this rainy season fade away without writing about the February storms--especially when they included a rare thunderstorm, and some were bigger than the December storm, which I did write about. So here it is May, the fourth average or wetter-than-average year in a row, and while the soil is already cracking due to the dry spring (although March was average with 6.3" of rain), things look good: the grasses are tall and there is still much green, and the now-disappearing flowers have been wonderful. And a pleasant surprise--the invasive deeply evil (in California, I'm sure it is nice where it is native) star thistle seems to be crowded out by other plants in many areas.

2025 water year total to-date is 47.87" as of the end of May.

Let's turn the clock back to February.

December 2024 Storm

On December 13, 2024, it started raining in the evening, and mostly stopped by 9am on Saturday December 14th.

The Nov 22nd and Dec 14th peak flows on San Geronimo Creek were the same: 1,105 cfs. Daily average flows were 423 cfs & 356 cfs, respectively. In November it took 8.75 hrs of 2.34" of rain (1/4" per hour) to get to peak from 113 cfs; In December it took 7.25 hrs of 2.32" of rain (1/3" per hour). These calculations use rounded times to the nearest hour for rain and quarter hour for flow & add an hour of lag time between rain and flow.

And then things got crazy.

Soil Science Joke

 Mono Basin Textural Triangle:

Note that in some lentic zones you might encounter loamy sand, sandy loam, sandy clay loam, or sandy clay. But there will be sand. In other areas you might encounter gravelly sand, cobbly sand, stony sand (not just near the dispensary), or bouldery sand.

Learning about equity from Monopoly

In recent months, when I played Monopoly with my kids, I realized how insidious the rule is that you don't have to pay someone who doesn't notice that you landed on their property. That rule encourages dishonesty and bad sneaky behavior--and could be responsible for the downfall of western civilization. Wait, never mind, the me-first capitalist mindset preceded Monopoly. But still, we don't play with that rule so as not to encourage dishonesty--at least while they are young and impressionable.

Paired-watershed Study

A paired-watershed study is conducted by using two similar watersheds and evaluating them for differences. One watershed is a control and the other has a management change applied to it. You can then measure the differences in flow and water quality exiting the watersheds, and have a pretty good idea of how the management change influenced any flow or water quality changes. Or in this case, with no management applied, the differences reveal natural differences in the watershed.

February 2020 was the driest February since 1864 in San Francisco. No rain fell there, as well as here in San Geronimo Valley (except for one weather station (WDAC1) that picked up 0.01 inches of precipitation on February 24th). December precipitation was 200% of average, however January was only 37% of average. February was 0% of average.

San Geronimo Creek peaked at a respectable 210 cfs in December--the highest December flow in 3 years, but less than 1/10th the peak flow in 2016--a good situation for Coho salmon redds, which got plenty of water but wouldn't have been washed out by flows high enough to scour the streambed. Since there weren't that many redds this year, a higher survival rate would be nice. January's peak flow was 70 cfs. February's was 6 cfs--perhaps the lowest February flow since 1864.

January 6th and 16th storms compared

The weekend of January 5-6, 2019 was a very wet one, and San Geronimo Creek reached 1,000 cfs for the first time since February 2017. 2018's peak flow was 753 cfs in March.

About a week and a half later, on January 16, 2019, there was another big storm--more well-advertised than the first, and windier, but not necessarily wetter. The storms were similar in many ways, but San Geronimo Creek peaked higher during the second storm, at 1,304 cfs.

Jan 6th storm, with day.hour on the x-axis.

Jan 16th storm, using the same day.hour as the Jan 6th storm, lining up
both peak flows between hours 17 and 18 on day 2. The Jan 6th peak was
at 5:30 pm and the Jan 16th peak was at 9:15 pm.

The graphs show the different patterns of rainfall and creek flow. The January 6th peak was broader, with a longer duration at high flows (over 400 cfs), however the January 16th peak had a longer duration at about 200 cfs. The antecedent conditions were wetter on the 16th--36 cfs 16 hours before the peak, vs. only 7 cfs 16 hours before the Jan 6th peak.

How do the total volumes compare? The typical pattern in San Geronimo Valley is for the Woodacre precipitation station (to the east) to be wetter than Mt. Barnabe (to the west), and that pattern held on the 6th. It was reversed on the 16th--Barnabe was wetter than Woodacre. Although the volume of runoff during the 16 hours prior to the peak was only 3% higher on the 16th, the magnitude of the peak flow on the 16th was 23% higher than on the 6th. The table below shows well how similar the storms were. Note the 6th was wetter in Woodacre, and the 16th was wetter on Mt. Barnabe.

Thalweg Surveys

Nerd Terrarium warning: Some terminology and concepts may be considered nerdy.

My first thalweg survey was in a Forest and Range Hydrology class at Cal Poly San Luis Obispo. We were surveying San Luis Creek downstream of downtown San Luis Obispo. I remember we reached a big, deep pool, and as a taller person, I got to go into it with the survey rod because I could reach the deeper parts while still standing. I remember climbing onto a big log and reaching out into the pool to place the rod in what I thought was the deepest point.

And that is what a thalweg survey is. The thalweg is the deepest part of a channel. If you are in a boat and not wanting to run aground, you would typically follow the thalweg. If you were walking up or down a stream, unless you wanted to get really wet, you would typically avoid the thalweg. A survey of the thalweg is done in order to produce a profile view of the creek--from which you can see the slope and pools and riffles.

On Sunday I walked downstream. It was a hot day, but I wasn't prepared to go in too deeply or do too much climbing over debris because I was carrying my sleeping son in a carrier on my chest. Nevertheless, I got farther than the debris-jam and pool that stopped me last year. Peak flows during the winter cleared the jam and deposited sediment in the pool.

The Last Bedload-mobilizing Peak Flow of the Season

Okay, you read the title and wonder, "why wasn't this in the Nerd Terrarium category?" Because that is a pretty nerdy title. I almost created that category when I wrote the title. I still may.

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.