Assessment of gravel transport characteristics of the upper Santa Ana River Scott Wright and Toby Minear USGS California Water Science Center Sacramento, CA

Information presented in this presentation is draft, subject to revision, and not citable A publication for submittal to peer-review is in preparation

Objectives/Questions  What is the gravel transport capacity of mainstem reaches? What are the effects of Seven Oaks Dam?  Which tributaries are important sources of gravel to the mainstem?  What is the fate of gravel in the reach currently occupied by sucker (below Rialto drain)? This study is funded by SBVMWD, with ~30% match from USGS

Methods  Review of previous studies and collection of existing data related to gravel transport  Field surveys of channel cross-sections, slopes, and bed sediment sizes conducted by USGS in 2013  Calculation of bed shear stresses for a range of flows and comparison with threshold values for gravel transport  Calculation of relative gravel transport capacity for a range of flows  Calculation of approximate gravel budgets (in vs out) for various reaches of the Santa Ana

Reach 1 Reach 2 Reach 3 Reach 4 City Cr Plunge Cr Mill Cr San Timoteo Cr Warm/Twin Cr Lytle Cr Reche Cr Seven Oaks Dam Rialto drain

Mainstem cross-sections and slopes

Reach 1 – Seven Oaks Dam to Mill Creek confluence 64 mm coarse gravel

Shear stresses exceed threshold values for gravel transport Seven Oaks dam dramatically reduces flows and bed shear stresses Reach 1 – Seven Oaks Dam to Mill Creek confluence

Reach 2 – Mill Creek to City Creek

Shear stresses mostly exceed threshold values for gravel transport Impacts of Seven Oaks dam are less due to inflows from Mill Creek Reach 2 – Mill Creek to City Creek

Reach 3 – City Creek to Lytle Creek

Shear stresses are declining downstream but still generally capable of gravel transport Impact of Seven Oaks continues to decrease due to tributary inflows Reach 3 – City Creek to Lytle Creek

Reach 4 – Downstream from Lytle Creek

Shear stresses continue to decrease downstream and the impact of Seven Oaks becomes even less due to tributary inflows Smaller floods unlikely to transport much gravel in this reach but larger floods could Reach 4 – Downstream from Lytle Creek

Mainstem summary with Seven Oaks without Seven Oaks Without Seven Oaks, stresses exhibit classical downstream decrease characteristics of depositional basins, driven by slope changes with Seven Oaks Seven Oaks has disrupted this pattern, particularly in the most upstream reach, but stresses are still mostly high enough to move gravel

Reach 1 Reach 2 Reach 3 Reach 4 City Cr Plunge Cr Mill Cr San Timoteo Cr Warm/Twin Cr Lytle Cr Reche Cr Seven Oaks Dam Rialto drain

Mill Creek Slope ~ 3.6%, width ~ 50 m

Shear stresses are above gravel transport thresholds, indicating potential for large gravel supply to the mainstem Results are similar to reach 1, both are very steep with very coarse bed sediment Mill Creek

City Creek Slope ~ 0.7%, width ~ 80 m

Computed stresses are much lower than Mill Creek (lower slope and smaller discharge) City Creek likely delivers significant gravel to the mainstem only during very large, infrequent floods City Creek

Lytle Creek Slope ~ 0.6%, width ~ 250 m

Shear stresses are lower than Mill Creek but higher than City Creek Could be a substantial source of gravel to the mainstem due to large channel size and discharge Lytle Creek

Other tributaries evaluated Warm/Twin Creek: Low slope, vegetated sandy channel with no evidence of gravel. Not likely to supply gravel to the mainstem San Timoteo Creek: Slope and drainage area suggest potential to supply gravel, but settling basins upstream from concrete channel likely trap most gravel Reche Creek: Visual evidence in the field of gravel supply to the mainstem, but small drainage area likely limits gravel supply in comparison to other larger tributaries

Tributary summary Mill, City, and Lytle have the greatest potential to supply gravel to the mainstem, particularly Mill (due to steep slope) and Lytle (due to large drainage area) Mill City Lytle

Mill Upper SA Plunge San Timoteo Reche City Twin Tributary summary Tributaries with high gravel supply potential have high elevation headwaters and large drainage areas

Gravel transport capacity Shear stress indicates the likelihood of gravel transport, but not rates Rates can be estimated based on the “excess shear stress”, i.e. the amount of stress above the threshold for movement We used the shear stress calculations to compute a relative measure of gravel transport rates, for comparison of mainstem reaches and tributaries, and construction of gravel budgets (in vs out) Rates were computed for each flow recurrence interval and integrated to estimate a long-term rate

Gravel transport index - mainstem 8 mm gravel 32 mm gravel Pre Seven Oaks, gravel transport capacity decreased downstream with decreasing slope Seven Oaks has reduced gravel transport capacity in all reaches, with the greatest reductions in the upstream reaches. This is typical of the downstream effects of dams on sediment transport

Gravel transport index - tributaries 8 mm gravel32 mm gravel Mill Creek has by far the greatest potential to supply gravel to the mainstem Lytle Creek also has relatively large transport potential City Creek likely does not supply much gravel to the mainstem due to relatively low slope and small drainage area Mill City Lytle Mill City Lytle

Reach 1 gravel budgets with Seven Oaks 0 40 without Seven Oaks unknown mm with Seven Oaks 0 2 without Seven Oaks unknown mm Because the gravel supply has been shut off by Seven Oaks, this reach should erode and coarsen (armoring) Pre-Seven Oaks, it was likely depositional

Reach 2 gravel budgets with Seven Oaks without Seven Oaks mm with Seven Oaks 10 5 without Seven Oaks mm Flows and gravel transport are reduced by Seven Oaks The reach remains depositional with Seven Oaks because of the large gravel supply from Mill Creek. Deposition rate is less post-dam Inputs: Reach 1 + Mill Cr Out: Reach 2 transport

Reach 3 gravel budgets with Seven Oaks without Seven Oaks mm with Seven Oaks 5 1 without Seven Oaks mm Flows and gravel transport are reduced by Seven Oaks The reach remains depositional with Seven Oaks because of the decrease in slope Inputs: Reach 2 + City Cr Out: Reach 3 transport

Reach 4 gravel budgets with Seven Oaks without Seven Oaks mm with Seven Oaks 2 1 without Seven Oaks mm Because of the distance downstream from Seven Oaks and inputs from tributaries (Mill and Lytle), the effect of the dam is apparent but not as substantial for this reach Inputs: Reach 3 + Lytle Cr Out: Reach 4 transport

Summary of findings  Flow regulation and sediment trapping by Seven Oaks dam is expected to reduce gravel supply and transport rates in all downstream reaches, with the effects decreasing downstream as tributaries “resupply” the mainstem with water and gravel  Post Seven Oaks, the four mainstem reaches studied should still have the capacity to transport a range of gravel sizes, particularly during infrequent floods  Mill Creek and Lytle Creek have the greatest potential to supply gravel to the mainstem reaches  Gravel supply to reach 4 (Santa Ana sucker habitat), along with ample in-channel gravel storage, suggest that the inset channel below Rialto drain can maintain a partial gravel bed, so long as sufficient wastewater discharge levels are maintained