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.
During major storms, we will post updates on Facebook and Twitter and Nextdoor neighborhoods.
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.
Is water being put into the many percolation ponds around the county to replenish the Ground Water supply? I see many of them are still empty (there is a very noticeable one at Almaden Expressway and 85 that seems like it should have water in it). Can excess water from our reservoirs be diverted to empty ponds or do we lose that water to the bay?
Hi Scott, thanks for your inquiry. Stormwater has a high turbidity, meaning it’s full of sediment and soil deposits. Putting this water into the percolation ponds would actually fill the pond with more sediment and inhibit the recharge rate. We have been releasing water from reservoirs to prevent the risk of flooding in immediate neighborhoods. Even though this water flows to the creeks and ultimately to the bay, natural groundwater replenishment occurs in the process. We have been managing a full recharge program and diverting water to percolation ponds since last year, and because of the community’s efforts to reduce water use, our groundwater storage has improved vastly. Also, a few years ago when the ponds were empty our crews thoroughly groomed the ponds, removing sediment and vegetation. This has allowed for water to seep quickly into our aquifers at a much faster rate than before.
I question your response regarding perk ponds. I don’t see any water at all going into the Campbell ponds. Recharge of the ground water is critical for our trees that draw on grown water and are beginning to die. Long range planning for water supply not only to our deep root trees is important to meet future population needs.