Lewis and Clark (Lewis especially) discovered a number of new plant and animal species on their trek to the Pacific and back. However, most of their scientific data went unpublished and remained unknown for many years. The web has a few offerings on these aspects, but I also recommend some books below:

See lists of the plants and animals discovered by Lewis & Clark.

See the Northern Prairie Wildlife Research Center (USGS) page on Birds and Mammals Observed by Lewis & Clark.

Dr. Mussulman's Discovering Lewis and Clark site offers a section on the Natural History aspects of the journey.

To learn more about the remaining plant specimens returned by Lewis and Clark, visit the Lewis and Clark Herbarium site of the Academy of Natural Sciences in Philadelphia, PA. Also see Peggy Edwards' collection of drawings of the Plants of the Lewis and Clark Expedition.

Also of interest is:
Flora and Fauna (recorded at Ft. Clatsop)

Related Books:

Lewis and Clark: Pioneering Naturalists
Paul Russell Cutright
Brompton Books Corp.
ISBN: 0-8032-6334-1
Widely available (such as at Amazon.com).

The Natural History of the Lewis and Clark Expedition
Raymond Darwin Burroughs (Editor), Robert Carriker (Introduction)
Michigan State University Press
ISBN: 0-8701-3389-6
Also widely available (such as at Amazon.com).

The Journals of the Lewis and Clark Expedition, Volume 12, Herbarium of the Lewis and Clark Expedition
Gary E. Moulton, William Clark, Meriwether Lewis
University of Nebraska Press
ISBN:0-8032-2931-3
July, 1999, $65.00

Volume 12 contains the most complete listing of the plant specimens catalogued by the Lewis and Clark expedition. All but one of the plants were collected by Meriwether Lewis, the most skilled botanist among the expedition's members. The collection, however, was nearly lost over the years due to its scattering among various botanists who intended to catalog the expedition's scientific discoveries. Fortunately, Gary E. Moulton tracked down the various specimens and here brings together 239 photographs of the vast array of flora that Lewis gathered. This invaluable volume will assist researchers and enthusiasts hoping to identify each plant's location, distribution, and use along the expedition's route.

The Lewis and Clark Collections of Vascular Plants: Names, Types, and Comments
Proceedings of the Academy of Natural Sciences of Philadelphia 149:1-64, 1999
James L. Reveal, Gary E. Moulton, and Alfred E. Schuyler, $10.00

Orders should be sent to:
Botany Department
Academy of Natural Sciences
1900 Benjamin Franklin Parkway
Philadelphia, PA   19103-1195

Payment should be in advance with order. Checks should be made payable to the Academy of Natural Sciences.

Also see the Academy of Natural Science's digital imagery set entitled The Lewis and Clark Herbarium.