Thanks for your comments and encouragement, Francisco.
This area is former swamplands and prior to that, probably an inland bay with saltwater intrusion. Over a long period of time, sediment filled in the area and than swampland plants such as tule, cattails, willows, etc. Over time, these plants developed peat soils which increased the elevation further and the areas would dry out during summers when river flows were low. Sediment from the rivers accumulated along the borders of the rivers (some natural but also some from a few years of hydraulic mining for gold perhaps 75 miles away until such practices were banned in the 1850s). Upon these sedimentary accumulations there were built hundreds of miles of levees to keep these areas dry during all months of the year (except for when the levees failed). Over 100 years of the land being drained (water seepage is pumped into the rivers) the organic matter decomposed and soil has subsided. My house is located at sea level and the fig orchard is about 5 feet lower. My farm's elevation ranges from about +5 to -10 feet. Organic matter level remains relatively high. Soils are fertile with the primary limitation being poor drainage in some areas though this is often controlled as much as necessary for the crops being grown. I have better soils which are already planted to chestnut orchard but this area should be fine for figs. After figs are a few years old there is a chance I will no be needing to irrigate them. If I dig down a few feet I will hit water. Actually, the water level can be much higher during the winter and that is why I planted the trees on ridges.