BOARD01926

• Board Meeting Date: 1/27/2004
• Description: ISF Section - Instream Flow Appropriations - West Antelope Creek
• Board Meetings - Doc Type: Memo

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slope and less sinuosity. This reflects the impact of the breached beaver dam, which
provided a steep gradient for flow.

Discharge Measurements

Discharge measurements were taken on June 6th and 7th, 2000, Data were collected
using English units, and these are tabulated in Appendix ill_ Discharge on June 6th was
1.26 ft3/s (0.036 m3/s) and on June 7th was 1.30 ft3/S (0.037 m3/s). The slight rise in
discharge in June 7th was probably due to the fact that the June 6th measurement was
taken in the afternoon and the June 7th measurement was taken in the morning. Reduced
rates of evaportranspiration during the intervening night before would allow for more
groundwater flow into the Creek and consequently higher flow in the morning.

Pebble Count

The Wolman (1954) method was used to estimate size of bed material in the channel. At
least 100 pebbles were sampled along several stream transects, and the b-axis
(intermediate axis) was measured to the nearest millimeter. Diameters of 2 mm or less
were recorded as 2 mm. Two pebble counts were made and the location of the each
pebble count is shown in Plate 1. The data collected are tabulated in Appendix IV. The
Dso for the two counts are 19 mm and 13.5 mm. This falls in the gravel or pebble range
(name varies with science discipline: 2 to 64 mm or 0.08 to 2.52 inches) and would be a
"4" stream type in the Rosgen (1996, p.5-26) classification,

Summary and Discussion

Using the stream parameters discussed above, this stream was classified using the Rosgen
(1996, p. 5-6) Stream Classification at a Level II. We classify the valley as a Type II:
moderate relief and moderate valley-side slopes. Vegetation is well-established on the .
valley slopes and floor. The valley compares well with photos shown in Rosgen (1996,
pA-13, his Figure 4-6a) of Valley Type II,

Width to depth ratio and entrenchment (flood prone width divided by bankfull width)
both suggest that this is a "B" stream. However, slope is steeper that the expected range
for this type of stream and would be classified as a "Ba" stream. Bed material is gravel or
pebble size, so the overall classification of the stream is "B4a". In the discussion of
longitudinal profile, we considered the upstream section and downstream section of the
stream separately_ The upstream section would classify as a "B4a" stream and the
downstream section would classify as a "B4b" stream, with the "b" indicating a more
gradual slope (see Table 1 caption). As previously mentioned, this reflects the impact of
the breached beaver dam on the upstream half, which produces a locally steeper slope.

There are several indicators of instability in this system:

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