New Oroville spills

How quickly they forget! Not many people in Ireland will be triggered by the word "Oroville" to be pitched six years back to Northern California when 180,000 citizens were told to leave their homes at very short notice because a dam[n] spillway had gotten all tore up and the emergency spillway looked likely to fail catastrophically and release a wall of water 100m tall to scour everything downstream. You, dear reader, may remember because I covered the story obsessively at the time but most folks have more pressing matter to occupy their minds - like the price of electricity here and now!

Dams and their reservoirs require active management to balance their complementary functions.

• Dams stop river flow allowing more controlled water supply downstream
• Water comes mostly in Winter but agricultural demand is mostly in Summer. Reservoirs keep the excess in reserve for demand six months later.
• Reservoirs also serve domestic water to wash behind ears, wash potatoes, wash cars and make coffee
  
• A big dam will have a big head of water behind it; so turbines can be en-coupled to generate hydroelectric power
• A big dam will create a large lake which has recreational / amenity value
  

Juan Browne at Blancolirio reports. The California DWR [Dept Water Resources] recently decided that Oroville Lake was filling up a little too fast for comfort given that a second atmospheric river was expected to dump A Lot of warm rain across the Oroville catchment. On Sat 11 Mar 2023, therefore, they opened the reg'lar spillway for the first time in 4 years to release 8,000 rising to 8,500 cfs [cubic feet per second = 225 cu.m /s]. See above: the white diagonal streak pluming out lower right is water cascading down that spillway. The white rectangle top left is the emergency spillway: reinforced, repaired, back-filled and backed up since the Big Ooops moment in Jan 2017. The terracing and roadways and landscaping are all new too. In all this single element of the power-generation + agricultural irrigation infrastructure has cost $1 billion to make it secure and functional with a judicious margin for the unexpected. 

225 cu.m /s is a lot of bathtubs but it's the drippiest of tiny drops in the context that Lake Oroville contains 4 billion cu.m. behind the dam spread over 6,250 hectares of surface. Then again there are a lot of seconds in a day and the atmospheric river wasn't due for 3 days. Putting all the numbers into a calculator says that DWR can lower the lake level by a meter in that half week. They don't want to lose their nerve and spill too much because excess will run off down to the ocean without growing a single almond and because the Feather River will erode banks and scour its bed if forced to carry too much flow. Then again, 2022/23 has been a snowy old Winter so far in N California: twice the average snowfall is waiting for Spring to fill the Lake with snowmelt.