Someone besides Nicholas Trist is sent to negotiate the Treaty of Guadalupe-Hidalgo, US gets additional territory, including Baja California, Baja is settled primarily by those from Confederate States, when the Civil War starts they form several units and attempt to march North on Sacramento, they are driven off by forces loyal to the Union, but are cut off from Baja and retreat into the mountains near lake Tahoe, several hundred strong. They quickly fort up and build improvised fortifications strong enough an attack without artillery support would be bloody on the Western shore. As part of a scheme to raid Carson City for supplies they use some extra timber build a gunboat armed with a six pounder and a couple swivel guns they managed to drag with them. The attack is a failure and they withdraw to their base on the Western Shore. To dig out the raiders the governor of Nevada has a larger vessel constructed armed with 4 12 pounder mountain howitzers, a 12 pound Napoleon and a 24 pound boat howitzer dragged from San Francisco. The larger Union vessel sails to the Confederate base near Emerald Bay and meets the Confederate vessel in combat, its explosive shells setting it ablaze, then turning the guns on the Confederate base and pounding it into submission
Best I got, unlikely but not too implausible I would hope