Distribution: Range extends primarily west of the Cascade-Sierra crest from western Washington to northern Baja, California. An isolated population is found in the interior-draining Mojave River of California as far into the Mojave Desert as Alton Canyon. Pond turtles have been collected on Vancouver Island, British Columbia, but these are believed to be escaped or released captives originally from the United States (Cook 1984).

Habitat: Typically riparian, most often living in sloughs, streams, and large rivers, although some may inhabit impoundments and irrigation ditches and other artificial bodies of water. In streams, pools are preferred over shallower areas (Bury 1972). Aquatic vegetation is almost always present along with basking sites, although the river bottom may either be rocky or muddy. Western pond turtles have been collected from brackish estuarine waters at sea level over 1800 m elevation in mountain streams (Stebbins 1954; Bury 1963).

Behavior: Turtles are generally active from late May to October (Bury 1972). In some areas of California, however, at east south of Shasta County, they may be active every month (Buskirk 1991). Hibernation occurs underwater, often in the mud bottom or a stream pool. During summer droughts, the western pond turtle may also bury itself in the soft mud bottom of a stream. The daily routine of the western pond turtle consists of a foraging period around sunrise (0530-0800) followed by a basking period (0800-dusk), with most basking occurring from 0900 to 1000 hr (Bury 1972). Basking usually occurs on rocks, logs, or on the bank, but may occur on tree branches that dip into the water from bank vegetation (Nussbaum et al. 1983). Basking rarely takes place by floating at the surface. Aggressive behavior may be initiated by a basking C. marmarata toward an approaching turtle (Bury and Wolfheim 1973). Other times are spent inactive in the water or foraging. While most foraging takes place earlier in the day, turtles may forage in the late afternoon or early evening during summer. During summer, these turtles move from pool to pool within the stream system.

Reproduction: Mating has been observed in the field in May, June, and late August and in captivity in late August and early September (Holland 1988; Buskirk 1991). The nesting season extends from late April through August, depending on the latitude; the peak period is late May to early July. Nests are dug either in the morning or evening and are generally located along stream or pond margins; however, nests have been found in fields over 100 m above and distant from the water. Rathbun et al. (1992) reported nesting in open, full-sunlight grassy areas with southern exposure. Clutch size ranges from 1 to 13 eggs (x = 6.3 for 15 observed nests). As many as two clutches a year may be laid. The natural incubation period is unknown, and may vary with altitude and latitude (Ernst et al. 1994). Laboratory incubation studies by Lardie (1975) and Feldman (1982) produced hatchlings in 73-81 days at 25-33 ^oC. Hatchling emergence may occur in late summer or fall, but Feldman (1982) and Buskirk (1991) suggested that some turtles overwinter in the nest and emerge the next spring.

Food habits: Known foods include: algae, various plants (including the pods of the yellow water lily), snails, crustaceans, isopods, insects (Coleoptera, Diptera, Ephemeroptera, Odonata, Orthoptera, Trichoptera, either as larvae, nymphs and/or adults), spiders, fish, and frogs (tadpoles and adults). Diets of adult males and females and of juveniles differ in prey size and proportions of prey items (Bury 1986). Males consume more insects and vertebrates while females consume more algae.

Populations: Populations are declining in southern California and over most of the northern range. Habitat destruction is given as the primary reason for declining populations (Brattstrom 1988, Brattstrom and Messer 1988). Today, only northern California and southern Oregon support extensive populations.