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Finding 4:  What explains Folsom Lake's artificial drought?

Finding  Summary

Initial two-part finding:

1.  It's not apparent why Folsom Lake is being maintained at an extremely low storage level.

2.  Folsom Lake storage management exhibits extremely high instability, a control issue known in aerospace engineering as a Pilot-Induced Oscillation (PIO).

This "Finding will be updated after seeking information from USBR Folsom Dam operations management.

There are major two issues in management of water storage in Folsom Lake: Capacity and unstable flow management.

As of March 2, 2017, Folsom Dam is 39% full. This is half of either of two averages in California:
    82% for 9 other resevoirs of similar or larger size
    77% for 46 California reservoirs of all sizes

  • Total inflow to Folsom lake since October 1, 2016 was 4.1 million acre feet.
  • Local consumptive use (discharge pumping) was 0.6% of outflow.

Graphic summary for all 46 reservoirs with daily updates on the web, supplied by DWR/CDEC
(California Department of Water Resources / California Data Exchange website):

California dams, percent full on 3/2/2017

Folsom Lake is being managed to produce artificial drought. Additionally, its storage management demonstrates very high instability, a circumstance known in aerospace engineering as Pilot Induced Oscillation (PIO).

Both circumstances are highly visible in the graph below, which compares Folsom storage with that of Bullard's Bar. Bullards is a reservoir on the Yuba River whose capacity is nearly identical to Folsom. In recent years both tended to fill at nearly identical rates.

Folsom Lake storage, 10/1/2016 to 3/2/2017

In this rain year, from October 1, 2016 to early March, watersheds of both the American and Yuba rivers have had abundant rainfall and snowpack accumulation. Folsom storage at the date of the graph was 600,033 acre feet below maximum capacity, 200,033 acre feet below the Top Of Conservation Storage (TOC STO). Capacity above TOC STO, 577,000 acre feet at this date, is the Flood Pool. The flood pool normally is reserved to control risk of downstream flooding.

The graph for Bullard's Bar is much more consistent with well-controlled processes for managing reservoir storage than is Folsom's. It also illustrates control without Folsom's huge oscillations in storage, which peaked at over 400,000 acre feet of capacity change in short periods.

As graphed for conditions on March 2, 2017 (Folsom) & Macrh 5, 2017 (Bullard's Bar):

ReservoirCapacityTop of
Conservation Storage
Actual storageAbove/Below
Folsom977,000 AF577,000 AF376,967 AF200,033 AF below
Bullards Bar966,000 AF796,000 AF775,542 AF20,458 AF below

Folsom's enormous oscillations in storage are typical of "pilot induced oscillations" in aircraft. These usually are due to an "impedance mismatch" between aircraft systems and the pilot. A historic example is described on sierrafoot.org for the case of Scott Crossfield's first landing of an X-15 research aircraft. Crossfield's PIO was due to control lag caused by a minor engineering problem that limited aircraft response rate to pilot control inputs.

Links to data updated daily on the CDEC web site:


is the California Data Exchange Center, operated by the California Department of Water Resources (DWR).