Site information:

10 mi S of Manteca on Two Rivers Rd, take S Manteca Rd to Trahern, turn right 1/4 mi, left on Two Rivers Rd to Two Rivers R.V. Park, 200 yards SE of confluence of San Joaquin and Stanislaus Rivers on N bank of Stanislaus

The location given is pretty vague.  The photos I took are in the general vicinity (N bank of Stanislaus, E of confluence) but I don't know how close I got.  Nothing remains of the site, as far as I know.

~37.665473,-121.240560

Plaque information:

Private plaque that isn't exactly about the battle, but related, is located at picnic area of Caswell Memorial State Park.

37.694180,-121.183866

Plaque text:

Estanislao's Stronghold

Near here in May 1820 Estanislao, the Indian chief of whom the river and county were named, defeated Sgt. Sanchez in a battle. Later the same month Gen. Vallejo returned with infantry cavalry and artillery burned the woods and routed the Indians. Estanislao escaped to Mission San Jose and Father Duran's protection, and was later pardoned by Gov. Echeandia.

Dedicated - October 19, 1974
Rededicated - October 12, 2013
Tuleburgh Chapter No. 69 by E Clampus Vitus

OHP description:

In 1829, the Governor-General of California directed Vallejo to punish the Cosumnes Indians for their raids on local ranches. The battle is one of the few fought in California in which cannons were actually used.

Registered 6/20/1935