40 
PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 64, Supplement 1 
In California, 431 aquifers (groundwater basins) have been delineated, underlying about 40 
percent of the surface area of the State. Of those, 24 basins are subdivided into a total of 108 sub¬ 
basins, giving a total of 515 distinct groundwater systems (California Department of Water 
Resources 2003). 
The top of the groundwater is called the water table or piezometric surface. Between the water 
table and the land surface is the unsaturated zone or vadose zone. In the unsaturated zone, mois¬ 
ture moves downward to the water table to recharge the groundwater. The water table can be very 
close to the surface (within a few meters), or very deep (up to a few hundred meters). In most Cal¬ 
ifornia regions, the water table is 3 to 30 m below the land surface; in some Southern California 
desert basins it is as deep as 100 m. Overall, ground water supplies one-third of the water used in 
California in a typical year, in drought years as much as one-half. 
Groundwater occurs everywhere in California, but it is not continuously focused along river 
corridors. Along fault lines and where there are abrupt changes in bedrock on steep slopes, 
groundwater can emerge as seeps and springs. The hyporheic zone is in sediments below surface 
waters in valleys. The hyporheos (“below flow”) is the biological community in the hyporheic 
zone. 
It is in California’s numerous valleys and intermontane basins that groundwater exists in the 
greatest quantity. The basins are like large bathtubs enclosed by the rocks of surrounding moun¬ 
tains. Over millions of years, these “bathtubs” were filled with many hundreds of meters of sedi¬ 
ment and debris that were carried into the basins by rivers and floods. In these so-called alluvial 
basins, groundwater fills small, often microscopic pores between the grains of gravel, sand, silt, 
and clay. Groundwater is also quite common in the hills and mountains surrounding these valleys, 
although in most cases not nearly as plentiful. In these rocks groundwater occupies practically 
every fracture and fissure below the water table. However, unless fractures are large and numer¬ 
ous, little water can be extracted. 
We have compiled biological data on 260 groundwater sites including 142 springs (38 biolog¬ 
ical sites); 52 streams and lakes (44 biological sites); 58 other hyporheic habitats below streams (49 
biological sites), and 8 wells (6 biological sites). 
We have many groundwater species in our database, but we omitted many of them from our 
species list here (Appendix 1) and included only phreatobites, which are obligate, generally troglo- 
morphic groundwater species. California differs from eastern states in having no obligate subter¬ 
ranean snails, either in groundwater or in caves. There is high endemicity in aquatic snails of the 
families Hydrobiidae and Physidae in California springs, but none are more than stygophilic. 
However, 32 phreatobites from 59 groundwater sites (Table 8) have been discovered in Cali¬ 
fornia. Of these, 23 species (72%) are single-site endemics. Phreatobite sites that were studied are 
concentrated in the Bay Area/Delta and Coast Ranges and in a few desert oases; most are not in 
karst. 
Stygobromus (Fig. 90) is a genus of crangonyctid amphipod crustacean that is an important 
component of many subterranean habitats in North America and Eurasia, with a total of 148 
described species. Twenty-five species of Stygobromus are known from California, seven of which 
are stygobites and 18 are phreatobites; six are undescribed phreatobites, most of which are single¬ 
site endemics. Stygobromus tahoensis and Stygobromus lacicolus inhabit the dark depths of Lake 
Tahoe, along with the eyeless Lake Tahoe flatworm, Dendrocoelopsis hymanae and the endemic 
flatworm, Phagocata tahoena. Lake Tahoe is 501 m deep and about 2 million years old. There are 
10 other stygobitic flatworms in California caves. The collection, preservation, and identification 
of flatworms is difficut. 
The asellid isopod Calasellus californicus occurs in 24 groundwater sites and Empire Cave, 
