The Carmel River watershed as seen from Pine Ridge 16 months after the Marble Cone Fire
The Monterey Peninsula gets almost all of its water from the Carmel River. The water is stored behind the Los Padres Dam in Cachagua and released to flow downstream into the Carmel Valley Aquifer. There it is pumped out of the ground by the California-American Water Co. and sold to the residents and businesses of the Monterey Peninsula.
When Los Padres Dam was built, in 1949, it held 3,300 acre-feet of water. 50 years later, in 1999, half of that capacity had already been lost to siltation. This very heavy rate of fill (an average of over 30 acre feet lost per year!) has badly compromised the dam’s ability to adequately supply the Peninsula with water.
It’s not the first time. San Clemente Dam, just upstream of the Carmel Valley Village, was built in 1921 with a capacity of about 2,000 acre-feet. Only 25 years later, its siltation problems were being cited as a reason why Los Padres Dam needed to be built. Today, in spite of Los Padres Dam blocking much of the sediment load upstream, San Clemente Dam is more than 90% silted in and completely useless as a water storage facility.
So what does fire have to do with this?
Well consider, for a moment, the fact that about a third of all the sediment now trapped behind the Los Padres Dam (an estimated 555 acre feet) arrived in a single year. 1978. The year following the Marble Cone Fire.
The Marble Cone Fire was followed by a wet winter that produced a maximum flow of 7,030cfs (cubic feet per second), as measured at Rosie’s Bridge in the Village. The Camel River stays a lot lower than that in most years, but 7,030cfs is still less than half the size of the floods that came down the river in 1995 (16,000cfs) and 1998 (14,700cfs) – and even those flows don’t compare with the Carmel River’s “Probable Maximum Flood” of 35,000cfs.
The big flows in 1995 and 1998 carried major loads of sediment into the reservoir, but didn’t approach what the much smaller 1978 flow carried. The difference, of course, was the fire.
So is the Los Padres Dam doomed to loose a third or more of its remaining (pitiful) capacity when the rain starts to fall?
Not necessarily. The Marble Cone Fire was an exceptionally hot fire burning massive quantities of snow-downed hardwood and fields of brush that hadn’t burned in many decades. And it was followed by a wet winter. The 1999 Kirk Complex Fires, in contrast, burning right in line with the estimated average natural fire interval (about 21 years), burned much more slowly and left far more brush and trees behind to hold the ground. The Kirk Complex was then followed by a relatively dry winter that only managed a maximum Carmel River flow of 3,160cfs – and no significant new siltation occurred.
The Basin Fire appears to have burned hotter – and it has certainly burned through more of the Carmel River watershed – than the Kirk Complex fire, but the short interval since the Kirk Complex and high relative humidities during the burn appear to have kept it much less intense than Marble Cone. The Basin Fire also burned earlier in the year, which may give the resprouting brush more chance to stabilize the soil before the onset of rain.
So just as with the threat of debris flows, the threat to the Monterey Peninsula’s water supply appears to depend on what kind of winter we have. It would probably take more than 1978’s 7,030cfs flow to give us a repeat of the 555 acre feet sediment disaster – and the odds are that flows won’t even reach that point, BUT … should we get the kind of rain we got in March 1995 or January/February 1998, Los Padres Dam’s usefulness as a water storage facility might well be ended, literally, overnight.
….. just another example of why the Monterey Peninsula’s near total reliance on water from a single small coastal stream has never been a good idea.
The Monterey peninsula cities are pushing for the desalination project which from what i’m understanding, will be located off the beach near Sand City. I guess they realized before even the Basin Complex fire, that something needs to be started soon. They must be afraid of loosing more of the two damns capacity sooner than anticipated. Thank you for this wonderful enlightening entry.
October 2008 – Accurate comparisons between the Marble-Cone and Basin Complex fires are difficult to obtain. The SEAT report on the Basin Complex fire says that 16% of the watershed above Los Padres Reservoir was burned in the M-C fire vs. 64% in the Basin Complex fire. But, Barry Hecht’s 1981 report on the effects of the M-C fire in the Carmel River watershed states that USDA Forest Service estimated that only 10% of vegetative cover remained in 42% of the basin (sounds like 42% of the basin was severely burned). In comparison, 10.5% of the basin was severely burned in the Basin Complex fire with a total of 49% moderately to severely burned, which is still very signficant.
Aerial oblique photos taken after the Basin Complex fire by the US Forest Service show that many of the south-facing slopes burned nearly completely. These slopes were vegetated primarily by fire-prone chamise and chapparal, so an influx of large woody debris into the Carmel River may not be as significant as after the Marble-Cone fire. But, the light, fluffy soil and ash left on the slopes by the recent fire will likely contribute sediment into the Miller Fork and Carmel River main stem (and ultimately to Los Padres Reservoir) for several years to come. A small amount of dry ravel (soil brought downslope by gravity) has already begun to fall into Los Padres Reservoir from adjacent, burned slopes.
About 60-70% of annual flow from the Carmel River watershed is produced in the basin contributing to Los Padres Reservoir. Base flows in the summer are likely to increase slightly for the next year or two due to a decrease in evapotranspiration, but probably not enough to offset the potential loss of storage in the reservoir this winter.
The loss of storage capacity at Los Padres Reservoir is now rising toward the top in watershed management issues.
Water Resources Engineer
Monterey Peninsula Water Management District