Coal-based generation in Nevada (from coal mined elsewhere) does exist, but the remaining plants are in the I-80 corridor, linked to Southern California only through a circuitous routing that's not commonly used -- that energy goes to Reno and Idaho instead.

Outside-of-California coal plants that significantly and regularly contribute to Southern California's energy mix are Navajo (in Page, AZ) and Intermountain (in Delta, UT). These are Los Angeles Department of Water and Power (LADWP)'s share and get mixed in through them. There's even a dedicated HDVC line [1][2] between Intermountain and Victorville.

The coal for Navajo is mined in Kayenta, AZ, and the plant's effects are multifaceted. It came about as a compromise when dams affecting the Grand Canyon were cancelled, and the mine and plant create jobs and lease payments for the Navajo Nation, but it also pollutes land and is used to power cities far away.

The coal for Intermountain is mined in Utah, Colorado, or Wyoming, depending on market forces.

LADWP has set goals to switch off of coal in the coming years, including considering converting Intermountain to natgas. Navajo's prospects are not good -- they're under regulatory and economic pressure, and conversion to natgas is not viable due to the lack of pipelines to the area.

[1] https://en.wikipedia.org/wiki/Path_27 [2] https://www.openstreetmap.org/relation/3392214