Vigil Network Site: Last Day Gully Near Hudson, Wyoming, U.S.A.
By William W. Emmett and Luna B. Leopold
U.S. Geological Survey, Washington, D.C. 20242
Last Day Gully is an example of a small ephemeral basin which has alternately degraded and aggraded in post-Pleistocene time, presumably in response to changes in climate. It terminates in a small alluvial fan on top of a 15-foot alluvial terrace in the valley of the Popo Agie River. The planimetric map of figure 1 illustrates the general configuration of Last Day Gully.
Because similar gullies in small basins are common and because their visual aspect alone does not indicate whether in the present climate these gullies are aggrading, stable, or degrading, this one was chosen for long-term observation as part of the Vigil Network.
This site is an ephemeral wash, or gully, about 1 mile northeast of Hudson, Wyo., NW 1/4, sec. 6, T. 2 S., R. 3 E., (lat 42° 55' 33" N., long 108° 34' 19" W.). It is included on the topographic maps published by the U.S. Geological Survey entitled "Hudson, Wyo.," scale 1:24,000 and "Lander, Wyo.," scale 1:250,000 (parts of both appear in this folder, figures 2 and 3). The main channel extends about 3,400 feet from the watershed divide to its end. The gully terminates in a semicircular alluvial fan. The gully bed is sandy silt and nearly free of vegetation. Vegetation adjacent to the channel consists of a mixture of low shrubs and grasses. Predominant shrubs are sagebrush (Artemisia tridentata Nutt.) and cactuses (Opuntia spp.). Grasses generally belong to the grama species (Bouteloua). Total vegetation cover varies but averages about 30-35 percent. The total area within the watershed is about 55 acres. The average elevation is 5,150 feet above sea level, and the relief between the watershed divide and the alluvial fan is 142 feet. Precipitation averages about 10 inches per year.
To reach the site of Last Day Gully, one may start at the center of the village of Hudson, Wyo. (see Hudson, Wyo., 1:24,000 topographic map, fig. 2), proceed east on State Highway 789 for three-quarters of a mile, and then turn north onto an unimproved side road. This road becomes a one-lane steel bridge crossing the Popo Agie River 200 yards from the junction with State Highway 789. After crossing the bridge one proceeds about 700 yards, leaves his automobile, and walks westerly along a fence, bearing N. 20° E. At a distance of 400 feet along the fence is the mouth of Last Day Gully, where the channel terminates in a low-angle fan. The alluvial fan and fence line are indicated in the upper left of the enclosed planimetric map of Last Day Gully (fig. 1). The permanent reference points along the stream, consisting of 1/2-inch diameter steel rods driven in the ground and protruding about 6 inches above the ground surface, are noted on the map by a small solid dot at each end of the lines marking the cross sections.
The principal measurements consist of 16 cross-channel land-surface profiles surveyed at locations specified in table 1 of this file (p. 17) and are also shown on the planimetric map (fig. 1). At the time of preparation of this file, four field surveys had been made; August 6-8, 1962, June 9-10, 1963, July 26, 1965, and June 18, 1966. Elevations from these surveys are listed in the next series of tables (tables 2-17). In addition to having bench marks, two of the cross sections were instrumented with 10-inch-long steel pins driven into the ground at given locations (sections A-B and E-D). Values of erosion can be determined accurately at these pins and are given in the tables for sections, A-B and E-D in lieu of elevations from annual resurveys (tables 15-16). It is emphasized that these 10-inch-long pins will not maintain their permanence if left unattended during periods of erosion that cause degradation exceeding the length of the pins.
In addition, a longitudinal profile of the main channel bed was surveyed over a distance of 3,575 feet beginning 150 feet below the fence line near the junction of the channel mouth and its alluvial fan. these data are found in table 18 of this file.
Other observations are being made, including depth of channel scour, height of floodflow, retreat of channel headcuts, and mass movement on slopes. Channel scour is measured by scour chains at stations 3+00, 6+00, and 9+00. The scour-fill record is incomplete because of the annual basis for resurvey. Scour, followed by slightly greater fill, is responsible for an overall aggradation of the channel bed. Height of floodflow is recorded on a crest-stage gage at station 1+50. Mass movement is being observed on two lines of pins installed near BM-D. these are indicated on the planimetric map (fig. 1) enclosed in this file. Annual surveys from the time of their installation in 1963 to 1967 shows no significant downhill movement of the pins.
The file of original field data includes a planimetric map (simplified reproduction, fig. 1) made by planetable survey of the channel in 1962 and black-and-white and color photographs taken from 1962 to 1966. Film negatives and the original planetable survey are on file with William W. Emmett, U.S. Geological Survey, Washington, D.C. 20242, U.S.A.
The following publications are partially devoted to information about Last Day Gully:
Emmett, W. W., 1965, The Vigil Network: Methods of measurement and a sampling of data collected: Symposium of Budapest, Internat. Assoc. Sci. Hydrology Pub. 66, p. 89-106.
Leopold, L. B., and Emmett, W. W., 1965, Vigil Network sites: A sample of data for permanent filing: Internat. Assoc. Sci. Hydrology Bull., v. 10, no. 3, p. 12-21.