The Lexington Basin was created by the San Andrea's fault complex millions of year ago. A groundwater basin was formed by ancient floods that deposited clay, slit, gravel, sand, and bedrock which became "aquifers", or underground water retention areas. The San Andreas fault zone is the dominant geological structural element in the area which separates the Santa Cruz mountain block (west side) from the Sierra Azul mountain block (east side). Subsidence along the fault complex combined with the uplifting of the Santa Cruz and Sierra Azul mountain block has resulted in the basin in which the Lexington Reservoir now lies.
There are three major climate zones or "ecotones" contained in the basin and surrounding mountains. The first is a mixed evergreen forest which is dominant on the southwest side of the lake with patches on the northeast facing slopes. Plants include madrone, tanbark oak, Douglas fir, maple, live oak and huckleberry, The second ecotone is a mixed riparian woodland which consists of dense groves of broad leafed deciduous trees found along stream banks in valleys and canyons. Look for willow, Big Leaf Maple, sycamore, black walnut, and oak in this area. It is also home to a redwood community which is dominated by the endemic Coast Redwood. This forest benefits from coastal fog and is found in north facing slopes. The last ecotone is the chaparral community which is typically sparse in trees and herbaceous species. This plant community includes chamise, sage, manzanita, toyon, and scrub oak.
The Lexington basin is believed to have been used by the Ohlone Indians for thousands of years as part of a trail corridor to the coast and Santa Cruz area. This trail was used for hunting access to seasonal camps and trade with neighboring tribes. The trail was later used by the Franciscan missionaries, connecting Santa Clara and Santa Cruz missions.
One of the first American settlers in the Lexington basin was a sea captain named Willard 'Julian' Hanks. He built a cabin in the Lexington dam area and in 1847, sold an equal share in water and lumber rights to Isaac Branham. Zachariah Jones (for whom Jones road was named) later brought out Hanks and Branham and built a thriving lumber business. It was from this area that much of the timbers was supplied for the miles of tunnels in the Almaden Quicksilver Mines.
Jones eventually sold his property to John Henning, who founded the town of Lexington. Henning named the town after his hometown in Missouri. Lexington became the largest settlement in the Redwood District which included Alma, Saratoga and Los Gatos. By 1877, Chinese laborers were clearing a railbed up Los Gatos Creek Canyon. The Southern Pacific Railroad soon reached Los Gatos and a narrow gauge rail was laid to the towns of Lexington and Alma. Leland Stanford (founder of Stanford University) funded the transition of the narrow gauge to standard gauge rail line and soon tunnels were bored through the mountains to reach Santa Cruz. The first of these was completed near Alma and is now flooded by the reservoir.
In 1919, Highway 5 connecting Los Gatos and Santa Cruz was completed with the road being paved in 1921. Competition with the automobile and the advent of W.W.II forced the railroad to close in 1940. This was the first blow to hit the towns of Lexington and Alma. In 1939, Highway 5 was replaced with a four lane highway and renamed Highway 17, bypassing many of the historic mountain resorts and communities. By the late 1940s the towns of Lexington and Alma were but shadows of their former selves.
The construction of Lexington Dam and Reservoir in 1952 was part of a countywide effort by the Santa Clara Valley Water District to address diminishing water supplies in the early part of the 20th century. Lexington was the last of eight reservoirs built during this time. The reservoir was appropriately named for the historic town of Lexington, which along with the village of Alma, was located where the reservoir now exists. The county added parcels in 1989 and 1990 to create today's 960 acre park.