Complex sentence: Its extensive watershed also reaches to the volcanic plateaus of Northeastern California and, historically, as far north as extreme south central Oregon when the now endorheic Goose Lake once experienced outflow.
Split into simpler sentences: Its extensive watershed also reaches to the volcanic plateaus of Northeastern California. Historically, its watershed reached as far north as extreme south-central Oregon when the now endorheic Goose Lake basin once experienced outflow.