Prior to the 1884–85 Geological Survey of the park, the Lamar was known as the East Fork of the Yellowstone River. During that survey, Geologist Arnold Hague named the river for L.Q.C. Lamar, then Secretary of the Interior, and a former slaveholder and author of the Mississippi Ordinance of Secession. The Lamar Valley, or the Secluded Valley of Trapper Osborne Russell and other park features or administrative names which contain Lamar are derived from this original naming. Osborne Russell in his 1921 Journal of a Trapper described the Lamar as follows: In 1869, the Cook–Folsom–Peterson Expedition encountered the Lamar River just upstream from the canyon section flowing into the Yellowstone and traveled upstream to the confluence of Calfee Creek where they camped on September 16, 1869.
The Lamar River, with its tributaries, is one of the most popular fishing areas in Yellowstone Park. The access is very easy and the cutthroat fishing is some of the best in the world. There are some rainbow trout in the river below the road bridge, but the primary fishing throughout the drainage is for Yellowstone cutthroat trout. Cutthroat trout in the Lamar offer good dry fly fishing with heavy hatches of caddis, pale morning duns, and large Green Drakes in July. Terrestrials are prominent in late summer. The Lamar and its tributaries are not usually fishable until about the second week of July, because of high water, so check conditions by contacting local fly stores. The National Park Service has made frequent changes to the regulations for the Lamar and for its tributaries including Slough Creek and Soda Butte, and in 2018 has made significant new changes. Anglers are now required to kill all non-native fish, including rainbow trout, brook trout, and identifiable rainbow/cutthroat hybrids throughout the Lamar drainage. On page 14 of the 2018 regulations they still say that if it has a red slash put it back, but that is clearly superseded by the region specific requirement that if a fish landed in the Lamar drainage is clearly identifiable as a hybrid then it must be killed, even if it has a red slash, with the caveat that "if you don't know, let it go." Another significant change to the Park-wide fishing regulations is that felt soles are no longer permitted on waders. Other Park-wide regulations, that continue from previous years, are that barbed hooks, lead weights, lead split shot, and live bait are banned.
Wildlife
While fishing along the Lamar River, one may see different types of wildlife. Along the river, there are pronghorn, bison, and bear within the area of the Lamar River Valley. Around the world, it is one of the most consistent environments to view wolves. There are two famous wolf packs in the area. The Lamar Canyon and Junction Butte wolf packs inhabit the area along the Lamar River. The river itself is home to cutthroat trout which attracts fishermen into the area. Birds in the area include osprey and bald eagle. The area along the Lamar River is often referred to as America's Serengeti.
Hiking
The Lamar River Trail is a seven mile long round-trip hiking trail. The trail's hiking difficulty is easy and the trip can be completed in one day. Along this trail, it is possible to see the extensive wildlife habitat the area has to offer. Even if one does not hike the entire trail, even only when hiking a few miles you will be near herds of bison along the trail. There are two great campsites, 3L1 and 3L2, along the trail within the area near Cache Creek. However, campsite 3L2 can be hard to access in the spring when the Lamar River is high, as it requires a ford in order to reach it. Along the upper sections of the trail one can find outfitters offering guided trips on horseback to go deep into the wilderness.
