Map of Washington Territory West of the Cascades, 1870 Source: David Rumsey Historical Map Collection
By Charlotte Jennings
Map of Boundary Disputes in Western North America Source: Missouri Society of Professional Surveyors
Although this map predates Washington’s statehood in 1889, the border between Washington State and British Columbia has remained very similar since the Treaty of Oregon in 1846. Washington State is the most northwestern state of the continental U.S., bordered by British Columbia to the North. Before it was Washington State, this area was part of Oregon Country, a contested area between British and American colonists. The Oregon Treaty codified the 49th parallel as the border between the United States and British North America on what had been interconnected Indigenous Coast Salish land for thousands of years. This treaty created a unique territorial landscape with exceptions to the 49th parallel division. Vancouver Island was originally going to be split between the U.S. and Canada, but the British refused to sign the treaty unless the island was an exception to the 49th parallel. This map’s upper left-hand corner displays the portion of Vancouver Island below the 49th parallel that almost belonged to the United States.
This map shows the northwestern edge of Washington State, west of the Cascade mountain range, one of the two major mountain ranges that define the state. The Cascade mountains run North-South and divide Washington into the wet and mild western side and the dry and semi-arid eastern side. The other major mountain range, the Olympics, lends its name to the peninsula depicted in this map’s lower half, home to the Hoh temperate Rainforest. The Hoh Rainforest receives an average of 135 inches (343 cm) of rainfall a year.
The Salish Sea, depicted here, is a marginal sea of the Pacific Ocean, encompassing the Strait of Juan de Fuca (named after the tectonic plate!), the Puget Sound, and the Strait of Georgia. The more than 40 Indigenous Nations in coastal British Columbia and Washington State are called the Coast Salish peoples from whom the name Salish Sea originates. The American islands in the sea are called the San Juan Islands and the Canadian islands are called the Gulf Islands. This map depicts Washington’s Olympic Peninsula with Canada’s Vancouver Island in the top left corner. Although the border between British Columbia and Washington is mainly defined by the 49th parallel (denoted by the dotted line), part of Vancouver Island and some of the Gulf Islands lay below the 49th parallel and belong to Canada.
Washington is also home to a pene-exclave named Point Roberts. 
A pene-exclave is a region within the borders of the country that it belongs to, but that can only be approached (normally by car) by traveling through another country. This 5 square mile (13 sq km) piece of land (in blue in the top right corner of the map) is on the southernmost tip of the Tsawwassen peninsula. The 49th parallel divides the Tsawwassen peninsula between the U.S. and Canada, isolating Point Roberts from the rest of the U.S. The only way to reach the rest of Washington by car is to drive 25 miles (40 km) through Canada, crossing the border twice! However, Point Roberts cannot be considered a proper exclave because you can reach the rest of Washington by boat without crossing into Canada. Point Roberts is one of a handful of examples of American exclaves along the Canadian border. This map is a fascinating example of the complications of national borders and how they can endure!
This map from 1870 depicts the Washington territory west of the Cascade mountains.