First incorporated in 1891, Pasco was the first of the three towns to make it's mark in the area. Next was Kennewick in 1904, and finally Richland in 1910. West Richland grew out of dissatisfied residents of Richland, wanting to be homeowners instead of renters of government-owned houses. Despite attempts by Richland to unify the community, they remained separate and eventually became incorporated in 1955.
Pasco was the largest city in the Tri-Cities because of the railroad station it provided. Pasco provided good farming land and industry and residents profited greatly from it Farming was the foundation of every sector of the economy in the early years. Farming still remains as a prominant industry in the Tri-Cities area and Pasco in particular.
Richland grew quickly as the larger of the three but then during the 1970s, Kennewick overtook Richland as the largest city of the three and continues to be to this day. The Columbia Center Mall was built on land newly incorporated into Kennewick, drawing growth to western Kennewick and south Richland.
Completion of the Interstate 182 Bridge in 1984 made Pasco much more accessible, fueling the growth of that city. During the 1990s, several major corporations entered the Tri-Cities, which helped to begin diversifying the economy. In 1995, a sixth public high school, Southridge High, was founded in south Kennewick.
The 2000s saw continued rapid growth as the Tri-Cities saw an increase of retirees from various areas of the Northwest. During this time all three cities flourished and grew significantly. Pasco became the fastest growing city in Washington State.
The Tri-Cities area continues to maintain steady growth and a stable economic climate due in large part to the American Recovery and Reinvestment Act of 2009 which dispersed jobs to the area.
The Tri-Cities are in a semi-arid climate, receiving an average of 7 to 8 inches of precipitation every year. While there are an average 225 clear days every year, these are mainly between April 1 and November 1. Temperatures range from as low as −25 °F in the winter to as high as 110 °F in the summer on average. The large Cascade Mountain Range to the west contributes to the semi-arid climate, which is far drier than the famously wet western side of the state.