With a break in the storms, we have a moment to reflect on the last week of events and to prepare for the weather pattern that’s headed our way next week.
The intense storm that reached its peak intensity on Sunday, Jan. 8 caused flooding in downtown Morgan Hill as water overflowed from West Little Llagas Creek. South of Gilroy, Uvas Creek exceeded its banks, forcing evacuations at two RV parks and ultimately closing U.S. 101. Many other small streams rose quickly, but beyond that in Morgan Hill and Gilroy, no major flooding was reported.
On Tuesday, Jan. 10, we monitored creek levels until the wee hours of the morning, as several reservoirs began to spill, resulting in high flows in the creeks below. Flows in Uvas Creek crept up to more than 7,000 cubic feet per second, close to the flows that caused flooding on Sunday. Los Gatos Creek also rose quickly overnight. Many parts of Vasona County Park were flooded because Vasona Reservoir rose more than 2 feet higher than the top of the dam’s spillway.
Storage in our ten reservoirs rose from 44% of their total capacity on Jan. 3 to more than 75% of capacity by Jan. 12. By Thursday, six reservoirs reached the level of their spillways, and began spilling water into the creeks below. These included Almaden, Lexington, Vasona, Coyote, Chesbro and Uvas. (You can see current reservoir levels here.)
Once a reservoir begins spilling water over its spillway, gravity takes over and we no longer have control of the quantity of water flowing into the creek below. The reservoirs are designed to contain high flows, but creeks below them could flood.
Over the next few dry days, reservoir levels are expected to drop as the amount of water flowing into them decreases and water is released over the spillway or through the reservoirs’ outlet valves. We are doing all we can to lower reservoir storage levels ahead of next week’s anticipated storms to reduce the chance of flooding.
We also have storage restrictions on five of our reservoirs due to seismic stability concerns and earthquake faults. We must release water from those reservoirs that exceed those limits as soon as possible. Currently, due to the heavy rain, Coyote, Almaden, and Calero reservoirs are above their seismic storage restriction, as set by the California Division of Safety of Dams, so we are releasing water from all three today to try and return these reservoirs to safe operating levels.
Anderson Dam’s outlet on Thursday, Jan. 12.
The weather forecast indicates more strong storms are on their way. With the ground highly saturated and many reservoirs full, there is a continued risk of flooding once those storms arrive. You can follow local conditions on our website, where we have dozens of gauges measuring rainfall, reservoir levels and stream flows.
If the National Weather Service issues a flood warning for your area, this means flooding is imminent or already occurring. Please stay clear of flooded areas. Never attempt to drive through flooded roads. For more flood tips, sandbag sites, visit our flood protection resources page.
Water supply outlook
On Thursday, the National Drought Mitigation Center issued a dramatically updated map for California, showing much of northern California out of the drought. This is certainly good news, but this map really shows broad-scale conditions. It’s not necessarily an analysis of local water supply availability. As an indication of rain and snow and other regional weather indicators, it does not consider local water supply.
Our local water supply outlook is not only based on rain and snow, but how much imported water we will be allocated in 2017, which we won’t know for a few more months, and the overall condition of our groundwater basins.
There is a high level of concern among regulatory agencies about the state of fisheries on the Sacramento River and in the Delta. As a result, there is much uncertainty about how much the state and federal water projects will deliver in 2017. We are currently updating our models, taking into account these recent storms, and will be presenting an update to our Board of Directors on Jan. 24, 2017.
While our water supply picture is improving, we should all continue to value water as a precious resource and never let it go to waste.