Site 4: Trailer Draw

GEOMORPHIC DESCRIPTION

The Trailer Draw site is located in the debris fan-dominated Lodore Canyon. Here, Trailer Draw debris fan restricts the channel along river left creating the Jack Springs rapid. Another debris fan along river right just downstream of the rapid further constricts the channel and intensifies its left turn bend. Below the constriction the river widens and slows creating backwater habitat on river left. An overflow channel flows into this backwater pool from upstream of the rapid. Geomorphic surfaces can be identified using sediment type, elevation, and vegetation. The bottom of Trailer Draw debris fan has been reworked into the pre-dam terrace by high flows before the completion of Flaming Gorge Dam and is a mixture of sand, boulders, and cobbles. Cottonwoods and box elders have become established on this terrace. The intermediate bench is mostly sand and vegetated by mostly tamarisk. This terrace is inundated approximately every 5 years. The post-dam floodplain is inundated annually and consists dominantly of boulders and cobbles built up from debris flows. Flows here are not high enough to transport large-sized clasts but are able to wash away finer sediments along channel margins. Downstream from Jack Springs rapid, an expansion bar island forms as the channel widens and flow velocity slows.

 

 

RIPARIAN VEGETATION

The riparian vegetation observed at Trailer Draw appeared to be locally variable, though the transect studied does not reflect this. The transect ran from the edge of the backwater habitat created by the debris-fan, to the edge of what appeared to be the pre-dam terrace (see geographic map). Results show the post-dam terrace was primarily dominated by marsh vegetation with a lower relative abundance of coyote willow (Fig. 1). The intermediate bench appeared to have a high relative abundance of coyote willow and a lower relative abundance of marsh vegetation (Fig.1). Coyote willow sampled along the transect appeared to be of a similar age class and were all of a larger relative size, possibly suggesting the age of the fan. The only recruited coyote willow and tamarisk were seen on the post-dam terrace of the expansion bar, though not represented in the sampled transect. No tamarisk was observed in the extremely dense vegetation of the intermediate bench (Fig. 1).

There appeared to be an extremely dense population of marsh vegetation in the transect region of the debris-fan, possibly because of the side channel which may still be reworking it (see geomorphology map). During periods of high flow the backwater area likely connects to the active channel inundating regions including where the transect was sampled and creating suitable soil moisture for wetland plants. The center of the debris-fan appeared to be of higher elevation and was therefore classified as a pre-dam terrace. Plants seen on this geomorphic surface were mature box elder, tamarisk and Russian olive.

WATER QUALITY

Temperatures in the backwater were higher than in the main channel. Turbidity is similar in the backwater and main channel probably because water sampling occurred after seining. Super-saturation of dissolved oxygen was not surprising due to the substantial aeration in the rapids.

FISH

Seine Collection Data:

 

General Comments:

Our angling efforts in this area continued to produce brown trout, most of which were unable to resist small rapala lures.  A “trophic cascade” rig, which consisted of a streamer tied about a foot above the rapala, yielded the largest fish of the day, a 20-inch plus brown, which was released. The river had changed to a less restricted meander, with beautiful undercut banks and increasing turbidity in the stretch following Swallow Canyon. The coloration of the fish were much more silvery and the contents of their guts housed a greater diversity of aquatic invertebrates than those fish caught in the upper reaches of the river.

AQUATIC INVERTEBRATES

Sampling sites were selected in slow backwater habitat as well as riffle habitat.  Substrate size and aquatic vegetation cover were estimated.

Habitat A - Backwater 
Substrate: 60% silt, 40% sand
Vegetation: Littoral submerged vegetation composed of Potamogeton crispus, Myriophyllum spp. and Elodea.
Diversity: 1.21

Habitat B - Riffle
Substrate: 5% sand, 5% gravel, 30% cobble, 60% boulder
Vegetation: ~80% Cladophora cover, some Myriophyllum
Diversity: 1.09

BIRDS, MAMMALS, REPTILES, AND AMPHIBIANS

We surveyed this site for an hour before noon during a slight breeze with between 25 and 50% cloud cover. This site, located in the Canyon of Lodore, is composed of a thickly vegetated hillside, with an abundance of sagebrush, juniper, and box elder. Birds in this survey area were identified by visual sighting or song recognition. Reptiles and mammals were identified by visual sighting or scat. Of interest was a dead great horned owl, whose body we discovered in a sprawled-out position on the bank adjacent to backwater habitat (Fig. 1). We also observed a Great Basin gopher snake eating an adult mottled sculpin (Fig. 2).