Carmel River Watershed: Garzas Creek and Carmel River Flow Duration Analysis
Garzas Creek has an upper and lower watershed, with a total 13.2 square mile drainage area (CCoWS). The upper watershed originates in the steep headwaters of the Santa Lucia Range, flows through a man-made lake named George Gordon Moore, and then joins into the Carmel River at the valley floor. The Garzas Creek gage ('GA') is located in a residential area near the base of the watershed at the bridge on West Garzas Road. The upper watershed of Garzas Creek flows through the newly formed Santa Lucia Preserve and upscale Rancho San Carlos development. In the late 1990’s, the Preserve gained the right to develop three hundred homes in two thousand acres of its twenty-one square mile area (Wilbur 2007). Lower Garzas Creek flows through Garland Regional Park and through a small residential area before entering the Carmel River. The two closest creeks entering the Carmel River east of Garzas Creek are Hitchcock and San Clemente Creeks. San Clemente Creek is the most similar to Garzas Creek in terms of length. Downstream of Garzas Creek, the Carmel River flows through Garland Park and the gage at the San Juan bridge. About twelve miles past the San Juan gage, the Carmel River flows under Highway 1 where a final gage monitors flow rate. At the mouth of the Carmel River is the newly restored Carmel River Lagoon and the Pacific Ocean.
The flow duration curve (FDC) is for the period from Oct 1997 through Sept 2008. The FDC for Garzas Creek indicates that at West Garzas Road, the creek flows only about 50% of the time. The general smoothness of the FDC indicates for most flow rates the regime is likely unregulated flow, however there is an inflection in the upper end of the curve at about 20 m3/s, which could indicate either flow regulation, or 'noise' in the curve due to a relatively short length of flow record. Although there do not appear to be dams or other major upstream systems interrupting the natural flow, there is a man-made lake that may weakly engage in flow regulation. There may be a weir at the lake, perhaps managed to fill the lake during high flow rates. This is just a guess and has not been substantiated.
Carmel River Gages at San Juan bridge and Highway 1
The FDC for the Carmel River at the San Juan bridge in Garland Park indicates perennial flow. There are no inflection points in the curve, indicating a natural flow regime in this section of the river. The term 'natural flow regime' should be interpreted cautiously because there are pumping stations, dams and wells upstream of this river section which undoubtedly affect the total amount of flow at San Juan bridge. During the low flow period (lowest 5%), there is a leveling of flow rate at about 0.03 m3/sec which may be low flows sustained by release from the Los Padres dam. Where the Carmel River crosses Highway 1 the river flows only about 65% of the time, indicating a dramatic drop in flow over the twelve mile course between the two gages. This drop in flow occurrence from 100% to 65% of the year can most likely be explained by percolation into the large aquifer underlying the Carmel River. Cal Am supplies much of the Peninsula's water by drawing from the Carmel River aquifer. This draw increases the amount of infiltration between the two gages as Cal Am draws from pumps at mid Carmel Valley below the San Juan gage. Both FDC curves developed from Carmel River gages approximately overlay one another for flows above the 20% level, indicating a condition when percolation is either: eliminated, minimal relative to the total flow, or offset by additional surface water inputs in the lower watershed. There is a slight increase in the FDC at Highway 1 compared with San Juan during peak flows, this is likely due to the addition of runoff from the lower part of the watershed, much of which probably only generates surface runoff during the wettest periods.
There are two large dams above both gages on the Carmel River, the San Clemente dam and the higher Los Padres dam. The San Clemente dam is filled with sediment, making it ineffective for water storage and unlikely to affect the flow regime. The Los Padres dam built around 1949 for 3000 acre feet capacity has silted to a 2007 level of about 1300 acre feet storage capacity (Brower 2007). The Los Padres dam has an estimated 40% storage capacity remaining, however water attenuation does not appear to influence the FDC to such an extent that certain flow rates are revealed by the FDC is being more frequent than others (i.e. flat points in the curve). A more thorough analysis would compare pre-dam with post dam FDC, yet flow records for the purpose of such comparisons do not exist.
(This passage is unreferenced and speculative in the context of the present analysis) Given current water use and pumping, one can imagine that in the absence of water retention and release by the dam, the river might dry further upstream and remain dry for a longer period. If Cal Am reduces pumping of the Carmel River to legal levels and there are no climate changes, the river should return to pre-1997 FDC conditions provided other developments have not influenced flows.
Comparing the three curves, the Carmel River flows at roughly an order of magnitude greater volume than does Garzas Creek for 20% of the time. When Garzas Creek is flowing at 20 m3/s, the Carmel River is flowing at 200 m3/s. so Garzas is contributing about 10% of the water to the Carmel River during this period. There are several other tributaries contributing to the Carmel River flow. Garzas Creek is dry for slightly longer than Carmel River at Highway 1 with no flow occurring for about 50% and 65% of the time respectively. Lower Garzas Creek dries during half the year while the Carmel River continues to flow above San Juan bridge at the juncture between Garzas Creek and the Carmel River. Garzas Creek continues to flow just above the residential areas in Garland Park. This flow is likely infiltrating into the Carmel River aquifer during the dry months. The most notable flow anomaly is between the two Carmel River gages at San Juan and Highway 1 bridges. There is a marked divergence between the FDCs at 2 m3/s where the FDC curve plummets steeply downward at Highway 1 and declines more slowly at the San Juan gage. The likely explanation for what happens to the water volume between these two locations is infiltration of flow into the aquifer,
The rate of infiltration probably increases during summer dry months as people irrigate their landscaping. Water flow in the Carmel River diminishes as water in the aquifer subsides. The Carmel Aquifer must be quite extensive because it was able to infiltrate 1420 acre-ft of water over the sixty days between 12/5/08 and 2/3/09 between the Robles Del Rio gage and the gage at Highway 1.
Based on visual comparison of flow duration curves the Garzas Creek / Carmel River system appears to exhibit watershed processes such as:
- Streambed infiltration
The Carmel River main stem evidences watershed process including
- Streambed infiltration
- P. Krone-Davis, pers. obs. over two summers
- P. Krone personal calculations based on USGS gage measurements at Carmel RA Robles del Rio and at Carmel River near Carmel between 12/5/2008 and 2/3/2009
- CCoWS. A more complete analysis was published in the Physical and Hydrologic Assessment of the Carmel River Watershed available at: http://ccows.csumb.edu/pubs/reports/CCoWS_CRWC_CarmAssPhysHyd_041101.pdf
- Wilbur, B. 2007. The Santa Lucia preserve is a place of natural and manmade beauty. Carmel: The lifestyle magazine of the central coast. p. 149-158. Available at: http://hunter-pr.com/uploads/in-the-press/Santa%20Lucia%20Preserve/CarmelMagSum07.pdf
- Brower, Bob. 2007. Available at: http://bobbrowerforwaterboard.blogspot.com/2007/11/state-of-los-padres-dam-november-2007.html