Pools in the Northwest Sacramento Valley Region are almost entirely the Northern Hardpan type. The majority of species occurring in the region are shared with the adjacent Northeastern Sacramento Valley Region. However, the soil conditions and geological substrate differ. The region includes primarily Northern Hardpan vernal pools and possibly Northern Volcanic Mudflow Vernal Pools to a limited extent in vicinity of Black Butte Reservoir. The individual pools are typically small and occupy regularly mounded, mima-mound topography.
Several complexes cover more than one section of land.
Because pools are small they typically do not hold water for long periods and lack many of the long inundation period species such as Tadpole shrimp. Exceptionally large pools up to several acres occur on the Stillwater Plains near Redding Municipal Airport. Subdivision of the old ranches and land grant areas west of Red Bluff, Corning, and Orland have taken their toll on pools in the region. Urban expansion of Redding has also caused substantial pool losses. Although the pools are on soils not typically used for intensive agriculture, recently thousands of acres of this area have been converted into Eucalyptus farms, and some have been converted into orchards.