Site 7: Sage Creek Confluence

GEOMORPHIC DESCRIPTION

At the confluence between the Green River and Sage Creek, a mixed alluvial/debris fan on river right substantially constricts the Green River against the adjacent bedrock wall of Whirlpool Canyon.  The channel constriction creates a pronounced fan-eddy complex characterized by sand-bedded separation and reattachment bars, and a small downstream expansion bar dominated by cobble-sized materials.  The post-dam flow regime appears unable to mobilize the coarse-grained debris fan materials.  Similarly, reduced flood flows have isolated substantial portions of the western fan surface, which is now colonized by sagebrush and rabbit brush.  Only small portions of the fan can be characterized as active floodplain deposits, suggesting that the post-dam flow regime is largely unable to mobilize debris fan deposited sediments.  Both the active floodplain and intermediate bench are colonized by tamarisk and willow.

 

RIPARIAN VEGETATION 

Sage Creek is an excellent example of distinct terrace formation. Transect 1 was constructed just downstream of where Sage Creek meets the Green River (see geomorphology map). It ran perpendicular to the Green River and spanned the lateral width of the post-dam terrace, intermediate bench, and approximately thirty meters of the extensive pre-dam terrace (see geomorphology cross-section). The post-dam terrace vegetation of transect 1 relatively equal assemblage of coyote willow and tamarisk recruits. However, the intermediate bench was extremely dominated by older coyote willow based on the increased diameter of their base. The pre-dam terrace was a combination of sparsely distributed coyote willow, sagebrush and rabbitbrush.

Transect 2 was placed further downstream beginning at the waters edge of an active sandbar (see geomorphology map). At the end of the sandbar, presumably the intermediate bench began as did a relatively high abundance of coyote willow (see geomorphology cross-section). A robust amount of herbivory was observed on coyote willow. Based on the height of the cuttings and sightings of ungulates in the area, it is possible that Bighorn sheep or mule deer were feeding on these plants. No tamarisk was sampled along the transect, nor were any recruits or saplings observed. However, large, mature tamarisk were observed in the upper reaches of the intermediate bench. The pre-dam terrace consisted mostly of rabbitbrush and sagebrush which is expected because of the lack of inundation to this terrace.

The transects selected did not, however, accurately represent the vegetation diversity in the region. Sage creek offers a great deal of moisture at an elevation away from the scouring flows of the Green river. Here we observed a very diverse species assemblage. Cottonwood, box elder, tamarisk, sagebrush, and marsh plants were all seen within or very near the boundaries of Sage Creek.

WATER QUALITY

Sage Creek is a small tributary that had an insignificant impact on water quality parameters of the Green River during our period of observation. Temperature and turbidity were much lower in Sage Creek, compared to the main channel of the Green River. Also, specific conductivity was much higher in Sage Creek most likely due to groundwater/baseflow inputs that are high in dissolved ions.

 

FISH

Seine Collection Data:

 

General Comments:

The backwater areas were continually dominated by the presence of small cyprinid species, which seemed to shoal non-exclusively with other fish of similar size.
Native fish seemed to be in good health but continually appeared in low numbers.

AQUATIC INVERTEBRATES 

Samples were collected from vegetation growing in slow water as well as in riffles. A third sample was taken from Sage Creek itself. Substrate size and aquatic vegetation cover were estimated.

Habitat A -  Slow water, littoral emergent vegetation
Substrate: 50% silt, 50% sand
Vegetation: Scirpus (bulrush) and dead tamarisk
Diversity: 1.8

Habitat B - Riffle
Substrate: 15% silt, 25% gravel, 50% cobble, 10% boulder
Vegetation: ~5% cover consisting of primarily Cladophora and dead tamarisk
Diversity: 1.49

Habitat C - Sage Creek, small clearwater tributary
Substrate: 70% silt, 30% sand
Vegetation: Scirpus, Acer negundo (box elder) detritus
Diversity: 1.65

BIRDS, MAMMALS, REPTILES, AND AMPHIBIANS 

We surveyed this site for an hour in the mid-morning during light air movement with less than 25% cloud cover. The survey area consisted primarily of sagebrush, juniper, and tamarisk. Birds were identified by visual encounters or song recognition. Reptiles were identified by visual sightings, while mammals were identified by both visual sightings and track recognition.

Muddy riverbanks and backwater habitat at this study area provided ideal surfaces for track identification. While we were at the site, a group of four male bighorn sheep headed our direction from approximately a quarter mile downstream (Fig. 1). They were slowly browsing and did not cover more than a few yards before our departure. At this site, we also caught a western terrestrial garter snake and a Great Basin gopher snake, both with prey located midway down their bodies. The woody vegetation provided habitat for the local avifauna; several birds (the spotted towhee, Lazuli bunting, and western kingbird) perched on dead junipers, while others (the Say’s phoebes and purple martin) perched on tamarisk.