Thread by @DustinMulvaney

The Power of the San Lorenzo River.. a short history.

Spanish settlers met the power of the river soon after the Portola Expedition and occupation. They built their 12th mission in 1791 along the river bank on rich fertile soil. After heavy damage from two floods in two years, they relocated the mission to an overlooking hill.

The new site they built above the river on Mission Hill is believed to be the location of Aulintak, a village of the Awaswas - speaking Uypi Tribe who lived with the river for thousands of years - today they are represented by the Amah Mutsun Tribal Band. nebraskapress.unl.edu/nebraska/97814…

There are accounts of major San Lorenzo River floods in 1847 and 1852, but the first documented in local newspapers is from the Great Flood of 1862.

Letter from Santa Cruz, January 14, 1862.

“The San Lorenzo rose higher than ever before known.”

Flood of 1869 - “The foot bridge across the San Lorenzo River is gone. Great fears are entertained for the safety of the new bridge on the same stream, near Santa Cruz”

Flood of 1871 - “The San Lorenzo rose far above the highest mark of 1862, washing over the bulkhead and flooding the lower portions of the city.”

1871 flood “The massive bridge over the California Powder Works was swept away and entirely destroyed.”

Powder works was built on land bought from paper mill destroyed in 1862. They used the power of the river to make explosive powder from guano for California’s mining industry.

Same 1871 storm, San Lorenzo Bridge in part destroyed, “about 240 feet of the bridge was carried away.”

An article from the Santa Cruz Weekly Sentinel says 23.5 inches of rain fell in 16 days in December 1871.

Flood of 1888 - “a large amount of driftwood came down the San Lorenzo River this morning and struck the bridge of the South Pacific Coast Railroad, the force of which added to high water, sweeping the bridge out of line four feet, making it impassible.” twitter.com/i/web/status/1…

1890 flood, again - “the river reached the highest point ever known.”

“20 feet of the upper bridge across the San Lorenzo River gave way, rendering the bridge impassible.. 100 feet of the railroad bridge on the Southern pacific, at the mouth of the river, fell into the water. twitter.com/i/web/status/1…

1906 “San Lorenzo quickly converted from peaceful stream into torrent.”

January floods swept away Loma Prieta lumber Co.’s mill at Hinckley Creek stranding 11 workers overnight; Late March rain contributed to a mudslide at the mill site after April 1906 SF earthquake, killing 9.

1907 Flood - Five Bridges of Boulder Creek Destroyed.. Bridges of Santa Cruz also menaced.

*Boulder Creek had some rough years 1904 to 1907. Curious how this wildfire, which burned for 14 months, and settler clear-cutting and grazing practices at the time contributed to the flooding and flood damage in 1907. #SantaCruzMountains

https://twitter.com/dustinmulvaney/status/1297766363457400832

Businesses and residents had long sought flood control, and after storms there would be renewed interest.

https://twitter.com/DustinMulvaney/status/1614397480942317568

In 1909, there was a push in the city for a bulkhead along the San Lorenzo river to be built by the railroads in exchange for track; this editorial “property holders, think!” emphasized how destruction to the watershed from deforestation was making flood risk worse.

But it wasn’t built, no funding.. so there were floods again in 1911, 1913, 1914, 1926..

December 28, 1931, San Lorenzo overflows banks for first time in 18 years.

*This 1931 storm was the first storm to shake the cement ship #SSPaloAlto. It moved in three feet that year. See 2023:

https://twitter.com/amylarson25/status/1611098077280210945?s=46&t=NBShAUpTI_RLTXD8Ss5WMQ

San Lorenzo on Worst Rampage of Century, February 1940.

February 1940 floods destroyed six of the nine bridges on Zayante Creek, a San Lorenzo tributary just upstream of big trees, and destroyed newly constructed cement bridges on Scott Creek and Liddell Creek.

March 1940, San Lorenzo threatens again.

Garfield St lowlands were evacuated several times in 1940 from raging San Lorenzo floods. City commissioned a report on flood control that year and Federal govt funded flood control surveys. Garfield St would’ve been to right in 12/31/22 photo, flow in photo ~6k cfs @ bigtrees.

February 1941, Raging San Lorenzo Sweeps Through Santa Cruz.

Proposals to reduce flood risk included removing a six acre island from the river mouth, building dams up the San Lorenzo gorge and further upstream, and building a wall along the river.

Flood of 1952 saw log jams as big trees gage hit a record at the time of 24,000 cfs.

The deadly 1955 flood flowed at an astonishing 30,400 cubic foot per second plus 8,100 cfs in Branciforte Creek. The confluence of those creeks was called the triangle of death. Downtown flooded in ten feet of water and nine died, including two inside their home on Garfield St.

https://twitter.com/dustinmulvaney/status/1612221077425623040

By 1958 the Army Corps committed to building levees.. the island was removed and the river straightened. Loch Lomond dam was built on Newell Creek, completed in 1960, and there was a proposal to dam Zayante.

The river height at big trees on the San Lorenzo was highest in 1982 than any other time on record, (though river at Ocean St was higher in 1955), but the levies held through the city. *The disaster from the 1982 storms would be the Love Creek Slide. pubs.er.usgs.gov/publication/pp…

Levees transformed the river. Big flows in 1996, 2006, 2017, 2022, 2023 stayed in channel through the city. The river is less prone to damage or injury, but the river also no longer supports the Coho and steelhead populations it once did.