First River, as the name suggests, is the first river ever discovered by humans of The Land. Its origin point is at the northern coast of First Land; it divides two mountain ranges, the First River Mountains on the west and the New River Mountains on the east (the two ranges would be considered a single range if not for First River itself falling between them). The precise origin is unknown, though the river flows north a brief distance before emptying into First Sea. The majority of First River, however, flows roughly south southwest, for approximately 2200 miles, before tapering off at the southern edge of First River Forest. Approximately 600 miles south of the coast, First River branches off in a secondary direction; this branch is called New River. Another river, Branch River, branches off from First River about 350 miles north of the southernmost tip of First River, about halfway through First River Forest. Branch River divides First River Forest on the west, from Valley Forest on the northeast.
The First River Mountains end about 1800 miles south of the point on the northern coast of First Land where First River meets First Sea. Nestled between First River, First River Mountains, and First River Forest, is the area that eventually came to be known as First Village. In LY 140, the inhabitants of First Village began construction of a bridge across First River, which was completed in 149. In 151, the area to the east of First River was given the name River Valley, as it was bordered by First River, New River, and Shoot River, as well as by the northwestern edge of Valley Forest (which was named after River Valley). It was in River Valley that striders were first discovered in 150.
First River has played an important role in Landian history for a number of reasons. It provided fresh water for the earliest generations of Landians, as well as providing fish. It was also the first body of water for which boats were designed to travel on, as well as providing the engineering challenge of building the Land's first bridge, and later the first water mill; all of these innovations were important to the development of Landian technology, and would later lead to even greater innovations, such as the construction of ships for sailing the oceans and seas.