Subsidence of peat lands of the Sacramento-San Joaquin Delta, California

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Introduction

At the confluence of the Sacramento and San Joaquin rivers in central California an area of about one quarter million acres, known as the Delta, is made up entirely of peat (fig. 1). These highly organic soils are said to comprise the second largest continuous body of peat land in the United States, exceeded only by the Everglades in Florida.

With minor exceptions, the entire Delta has been reclaimed from tule swamp. In its present high state of cultivation, it produces farm products with an annual value of more than 25 million dollars.

The area consists of a number of tracts and islands, each organized into one or more reclamation districts under state law for the construction, maintenance, and operation of levee and drainage systems. There are more than 80 reclamation districts in this area.

Around the borders of the Delta the peat soils merge gradually into the adjacent mineral soils, or feather out in shallow depths of peat overlying mineral soils. The center of the Delta is composed of islands which are entirely surrounded by the interlacing channels of the two major rivers. In portions of this area the peat is 30 feet or more in depth. All of the channels surrounding the islands are navigable to barges and other freight-carrying craft.

Cultivation in the Delta region dates from the passage of the Swamp and Overflow Act of 1850, when title to these lands passed from the federal government to the state. The first levees were low dikes, truly handmade by Chinese labor with wheelbarrows and shovels. The early reclamation, therefore, did not penetrate deeply into the heart of the peat lands.

Prior to reclamation, the entire area was covered with a dense growth of tule (Scirpus lacustris), with a fringe of willows and other woody plants along the slightly higher stream banks, where the soil contains the greatest amount of mineral material. The surface of the area was at zero, or sea-level, elevation and was inundated at high tide or at flood stage in the rivers.