What’s worse than months or years without rain? Rain, after months or years, at least under some circumstances.

For instance … it gets try, plants become vulnerable to fire. Fires happen denuding the dry landscape. Then it rains, and you get more severe floods together with landslides. You know the story because for years this has been the pattern in California.

But there is another roughly similar, or at least analogous, problem that is now being discussed. The levees that are mean to keep floodwaters contained in California were already in fairly bad shape. Prior to the drought, a significant number of levees were known to be at risk of failure should they actually get used. Many are thought unable to handle earthquakes as well.

But with the drought, several factors have probably made the levees weaker. This is an ongoing process and will continue as long as the drought continues.

From a letter to science, “Drought threatens California’s levees” by Farshid Vahedifard, Amir Aghakouchak, and Joe Robinson,

Prolonged droughts undermine the stability of levee systems by increasing water seepage through soil, soil cracking, soil strength reduction, soil organic carbon (SOC) decomposition, and land subsidence and erosion . The sand-clay mixtures, which form the body of the levees and consequently the entire structure, can lose a substantial amount of strength under dry conditions. Furthermore, levees in California are built on peaty soils, and the extreme drought leads to greater SOC decomposition in these soils. A large amount of the global carbon stock is found in peaty soils, and ~25% of this estimated stock is predicted to diminish under extremely dry conditions. Oxidation of SOC under a prolonged drought can also accelerate land subsidence. In fact, 75% of the land subsidence across California is accredited to oxidation of SOC. Land subsidence can increase the risk of water rising over the top of the levees.

This happened in Australia. Remember the big flooding a couple of years back? Some of that was made worse by levees failing, and those levees had been weakened by prolonged drought. So this is not theoretical.