A Look Inside the Data: Decreased Water Flow in California

Hello and welcome back to The Deluge!  

Today’s dataset is a table from Chapter 1 of the Report of Sacramento-San Joaquin Water Supervisions for Year 1943 from the California Water Documents collection. This dataset reveals interesting information that supports the work that environmental conservationists perform across the globe.

Table 5 gives a comparison between the 40-year and 50-year mean of the full natural water flow for all major streams entering the Great Central Valley via the Sacramento and San Joaquin rivers. The Great Central Valley is California’s largest agricultural hub and provides over half the fruits, vegetables, and nuts grown in the United States. 

Comparison Table

Table 5, Ch. 1 from Report of Sacramento-San Joaquin Water Supervisions for Year 1943, page 11

The following data analysis highlights the significant decrease of water flow to the Great Central Valley of California.  


The left side of the table shows the river and station where full natural flow was measured. The 40-year mean is between 1889-90 to 1928-29 and the 50-year mean adds a decade to the calculations. These water flow values are in acre-feet. An acre-foot is a unit of volume equal to the volume of a one acre sheet of water surface area to the depth of one foot, 43,560 cubic feet in total.    

The most shocking observation about this table are the percentage changes between the 40-year and 50-year mean of water flow. Ideally, the numbers would stay the same, but only two stations on the Kern and Kaweah rivers saw a 2% decrease in water flow. Most rivers saw a 4-7% decrease in natural flow. The Sacramento River experienced a mean natural flow decrease of 1,337,000 acre-feet. This data is highly valuable for conservationists monitoring California’s gradual incline towards disaster level drought territory.  


This table shows a significant decrease in water flow over the span of a decade. The negative impact that a century of irrigation played on the rivers and streams here in California is clearly evident.