The levees around Natomas were designed to handle the historical “flood of record” which was the 1907 and 1909 floods on the Sacramento River. Another large flood event occurred in 1937 which the system safely passed with only minor problems. Again, in 1955 an even larger flood roared through the Central Valley around Christmas. The Natomas levees held with some minor sloughing along the Sacramento River near the Sacramento/Sutter County line. However, as a result of this flood, the Army Corps of Engineers raised the Natomas Cross Canal and Pleasant Grove Creek Canal levees as much as two to three feet in anticipation of future even larger flood events. In addition, by 1955 Folsom Dam was operational which provided additional flood storage capacity along the American River on the District's southern flank.
For the next thirty years, smaller floods came and went without incident until February 1986. During a 10 day period starting on Valentine's Day 1986, over XX inches of rain fell on the Central Valley resulting in the current “flood of record” on the Sacramento and American River basins. The District levees were seriously challenged as large areas experienced significant seepage causing the backside of the levees to erode. Approximately 10,000 feet of levee along the Sacramento River west of the airport sustained major sloughing. A flood fight was initiated by the District and eventually, the Army Corps of Engineers assumed responsibility. Were it not for the quick action by the District and the extended efforts by the Corps it is likely a levee failure would have occurred somewhere along the Sacramento River during that flood.
Following 1986, levee repairs were constructed by the Corps of Engineers to remediate the levee seepage problem. This work was completed in the early 1990s. In addition, a joint powers authority known as the Sacramento Area Flood Control Agency or SAFCA was formed to develop a comprehensive flood control project for the entire Sacramento area. On New Year's day 1997, a flood eerily similar to the 1986 event hit Sacramento cresting at almost the same river level. Because of the improvements constructed by the Corps several years earlier, the Natomas levees safely passed this flood with some seepage but little to no levee damage. However, this flood caused levee failures elsewhere in the Sacramento River system and has awoken a concern about potential “under seepage” issues and their potential to cause a levee failure in the future. In addition, since these two very large flood events occurred within such a short time frame, experts now believe we should plan our flood control system and levees for even larger floods; especially levees protecting urban areas where damages would be extensive and the potential for loss of life high. This was further ingrained in our planning efforts by the flooding in New Orleans as the result of Hurricane Katrina in 2006 and the resulting catastrophic damages and lives lost.
Efforts are currently underway to address both these potential risks with the goal of providing Natomas at least 200 year level of flood protection (a 0.5% risk of flooding in any given year) and looking for opportunities to improve the system even beyond this level; particularly as the urbanization of the basin continues.