In 1851, surveyors placed a marble obelisk on a mesa overlooking the Pacific Ocean, which demarcated the United StatesMexico boundary line. Tourists flocked to the region alongside land speculators who envisioned upscale hotels, resorts, and spas. Two decades later, an East Coast journalist, William Smythe, established a utopian agricultural colony in what is today San Ysidro. Tourists began to cross the border in droves when Tijuana earned the reputation as vice city. Racetrack, saloon, and gambling house employees settled in San Ysidro, while ranchers in the Tijuana River Valley bred horses for the racetracks. Dairy and vegetable farmers also moved in, taking advantage of the year-round mild weather. By the 1970s, suburban development and greater restrictions to the flow of people at the border meant the area became a predominantly Spanish-speaking community. The Port of Entry at San Ysidro also became the largest in the world, accommodating over 47 million people annually.

The Tijuana River Valley Historical Ecology Investigation synthesizes hundreds of historical maps, photographs, and texts to reconstruct the ecological, hydrological, and geomorphic conditions of the Tijuana River valley prior to major European-American landscape modification. How did the valley look and function before there was the state of California, the city of Tijuana, or an international border? What habitat types and wildlife were found there? How have these habitat types and the physical processes that shaped them changed over time? And finally, what can the valley's ecological past tell us about its present and future? In answering these fundamental questions, this richly-illustrated study provides scientists, managers, and residents in the valley with information designed to support and inspire ongoing management and restoration activities.