Assessment of gravel transport characteristics of the upper Santa Ana River

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

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

3. 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

4. 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

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

6. Mainstem cross-sections and slopes

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

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

9. Reach 2 – Mill Creek to City Creek

10. 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

11. Reach 3 – City Creek to Lytle Creek

12. 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

13. Reach 4 – Downstream from Lytle Creek

14. 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

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

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

17. Mill Creek Slope ~ 3.6%, width ~ 50 m

18. Mill Creek 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

19. City Creek Slope ~ 0.7%, width ~ 80 m

20. City Creek • 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

21. Lytle Creek Slope ~ 0.6%, width ~ 250 m

22. Lytle Creek 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

23. 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 supplyin comparison to other larger tributaries

24. Tributary summary Mill 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) City Lytle

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

26. 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

27. Gravel transport index - mainstem 32 mm gravel 8 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

28. Gravel transport index - tributaries 32 mm gravel 8 mm gravel Mill Mill Lytle Lytle City City 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

29. Reach 1 gravel budgets 8 mm 32 mm 0 0 unknown unknown without Seven Oaks with Seven Oaks without Seven Oaks with Seven Oaks 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 300 40 30 2

30. Reach 2 gravel budgets 8 mm 32 mm 40 10 400 150 Inputs: Reach 1 + Mill Cr Out: Reach 2 transport without Seven Oaks with Seven Oaks without Seven Oaks with Seven Oaks 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 200 70 20 5

31. Reach 3 gravel budgets 8 mm 32 mm 20 5 200 80 Inputs: Reach 2 + City Cr Out: Reach 3 transport without Seven Oaks with Seven Oaks without Seven Oaks with Seven Oaks Flows and gravel transport are reduced by Seven Oaks The reach remains depositional with Seven Oaks because of the decrease in slope 50 20 3 1

32. Reach 4 gravel budgets 8 mm 32 mm 2 90 60 5 Inputs: Reach 3 + Lytle Cr Out: Reach 4 transport without Seven Oaks with Seven Oaks without Seven Oaks with Seven Oaks 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 40 30 2 1

33. 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