SJRA Releases Third Part of Sedimentation Study
In late December 2023, the San Jacinto River Authority (SJRA) released the third part of its massive, years-long sedimentation study. SJRA labeled the report “Technical Memorandum 3: Annual Sediment Supply and Storage.” Only seven more steps remain in the 10-step effort.
The Larger Context
The ten steps include:
- Inventorying available existing data (January 2023)
- Characterizing the subwatersheds (January 2023)
- Quantify sediment supply and storage volumes in three representative subwatersheds (just released)
- Sediment transport modelling
- Extrapolate subwatershed data to entire basin and prioritize sediment sources for investigation
- Investigation of hotspots to prioritize management opportunities
- Conceptual solution and strategy development
- Identification of potential partners
- Identifying sources for technical and financial assistance
- Synthesize previous findings into final plan and publish
SJRA applied for a TWDB grant to study sedimentation in July of 2020 and accepted the grant in July 2021.
Sediment Supply and Storage Methodology/Findings
The study team collected data from three watersheds in the Upper San Jacinto River Basin: Winters Bayou/East Fork, Peach Creek/Caney Creek, and Walnut Creek/Spring Creek (also referred to as the Willow Creek area in the study). They collected samples, measured isotope concentrations, and performed a chemical analysis to determine a “fingerprint” for sediment originating in those areas. Investigators then compared fingerprints of sediment taken from Lake Houston to identify the likely source or origin.
They also measured erodibility of river banks by assessing such factors as bank height, angle, and material; root depths and density; proportion of the bank covered by vegetation, rocks, logs, etc.; and stability of soil strata.
High-Level Finding
Matt Barrett, SJRA’s Water Resources and Flood Management Director, said, “The key data obtained in the efforts described in TM 3 was the determination of where sediment appears to be originating in the sub-watersheds (i.e. upland runoff vs. streambank erosion).”
“Our findings are that sediment appears to be originating primarily from eroding streambanks.”
Matt Barrett, SJRA Water Resources and Flood Management Director
The study did not directly examine the relative contributions of clear cutting, construction practices and sand mining to sedimentation.
Other Findings
Regardless, the study yielded interesting information about the soil in each area sampled.
Erosion rates (ft/yr) at the three locations varied by more than 3X. When measured in tons/ft/yr, they varied by 4X.
At all three streambank sites, samples were composed of varying proportions of sand, silt, and clay, with sand the most prevalent component. Samples taken from floodplains varied more, but bore a resemblance to the corresponding streambank samples.
Samples from the East Fork San Jacinto stream bank and floodplain had more than 50% fine-grained composition, i.e., silt and clay. See table below.
Study authors say, “…the relatively high proportions of fines in the East Fork … samples indicate(s) appreciable upland sediment contribution, consistent with the Pb-210 data.” (See Page 26). Pb is the chemical abbreviation for lead.
Highest Levels of Lead Coming from East Fork
Chemical sampling of Lake Houston sediments indicates that the highest percentage of lead, a naturally occurring element, was found in the East Fork Mouth Bar.
Looking farther upstream at three possible sources of the lead (upland areas, stream banks, floodplains), the authors found that floodplains and upland areas contributed more than streambanks.
It’s impossible to determine the exact point of origin of the lead at this point in the study, say the authors.
The report never mentions Colony Ridge per se. Investigation of “hotspots” won’t happen until Step 6. (See list above.)
However, in my opinion, more study is merited to see whether the lead came from the Colony Ridge area. It has been the largest land-disturbing activity in the East Fork watershed in recent years.
Widespread destruction of wetlands in the massive Colony Ridge development may have contributed to the high lead levels. Wetlands can be pollution-catching powerhouses, according to a University of Waterloo study.
Head Scratchers
In the meantime, I’m wondering how the SJRA’s sedimentation study could have omitted the West Fork San Jacinto as a sampling site. The study does not explain. Barrett said only that they couldn’t afford to sample every watershed.
Yet the West Fork has the largest concentration of sand mines in the region (more than 20 square miles in a 20 mile reach of the river).
However, the report never mentions the phrase “sand mines.” In one paragraph on page 31, it does mention APOs (aggregate production operations). But I had to laugh. It said that ReduceFlooding.com reported “several purported breaches” of APO berms on the West Fork.
In this context, “purported” means “suspected” according to Webster’s Third International Dictionary.
Sampling Concern: Avoiding the Obvious?
I guess they didn’t see the hundreds of photographs that I took of mine breaches or mines pumping their waste into the river. Nor did they read the TCEQ investigation about a 56-million-gallon release of sludge that turned the West Fork white.
Neither does the report mention the terms “development” or “land clearing” – recognized by the EPA as major sources of sedimentation.
That contributed to this sludge fest on the West Fork.
A friend of mine with a PhD in market research once told me that good research often confirms what in retrospect should have been obvious. But the SJRA study isn’t even sampling the obvious.
No telling where their sedimentation study will go at this point. This reminds me of the SJRA’s sand trap study. It recommended relying on a sediment gage upstream from all the sand mines.
Are they avoiding the obvious in their choice of areas they sampled?
Ultimate Goal
When I asked Barrett what he hoped the study would accomplish, he said, “The ultimate goal is to develop conceptual sediment mitigation solutions and implementation strategies. These could be infrastructure projects (stream restoration, sand traps, etc.).”
He continued, “Or they could be best management practices that could be recommended to appropriate government bodies (recommending riparian buffer or enhanced stormwater protection plan requirements that a county could implement, for example).
“These solutions will be included in the Sediment Management Plan, which will provide a ‘menu’ of projects that could be implemented to help mitigate the negative impacts of sedimentation.”
I’ll withhold judgement until we get to the end of this road.
For More Information
Read the entire 66-page Technical Memorandum 3. And review other documents currently posted on the SJRA Sedimentation Study website.
Posted by Bob Rehak on 1/16/24
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