Site 8: Big Island

GEOMORPHIC DESCRIPTION

This small mid-channel island just south of Big Island in Island Park and just downstream of the termination of Whirlpool Canyon is dominantly cobbles on the floodplain of the upstream side and grades into an intermediate bench that is dominantly sand. The island is probably inundated annually on the upstream side and every 5 years on the downstream side. The upstream side of the island has young tamarisk, cottonwood, and willow plants while the downstream side of the island is vegetated by older tamarisk plants. The floodplain along the downstream edge of the island is dominantly fine sediment or cobbles with no vegetation.

 

 

RIPARIAN VEGETATION

Riparian vegetation on Big Island appeared diversified longitudinally (see geomorphology map). One transect was applied spanning the length of the island. We observed two geomorphic surfaces including a post-dam terrace and an intermediate bench (see geomorphic cross-section). On the active upstream floodplain, coyote willow, tamarisk and cottonwood appeared to be recruiting. Relative abundance ranged from high to low respectively (Fig. 1). As we progressed away from the post-dam terrace and into the higher elevation intermediate bench, relative abundance of coyote willow continued to increase while relative abundance of tamarisk appeared to decline. Maximum diameter appeared to increase suggesting an increase in age diversity on the intermediate bench. The substrate also changed from coarse grained to sand along the transect.

On the downstream, northern side of the island, herbivory of both tamarisk and coyote willow was observed. Based on the height and location of the cuts, it is possible the cutting was performed by beaver.

WATER QUALITY

Big Island is surrounded by a larger main channel of the Green River and smaller side channel producing a lot of backwater habitats. These areas were slightly warmer than the main channel. Turbidity was slightly lower in the side channel perhaps due to slower water velocity and subsequent settling of particles that were suspended in the steeper gradient Whirlpool Canyon just upstream.

FISH

Seine Collection Data:

 

General Comments: 

Upon leaving the rock walls of the canyons the river widened greatly and backwater habitat became much more abundant.  A wider array of species was observed in this reach of the river. Areas where sampling occurred included shallows bordering the main channel and more stagnant deep pools in backwater areas. The presence of mosquitoes and black flies was bothersome at times.  Seining was performed during daylight and nighttime hours. Mountain whitefish were more prevalent during nighttime seining.

AQUATIC INVERTEBRATES

Sites on Big Island were located along the main channel of the Green River, one was a backwater pool and the other was a riffle.  Substrate size and aquatic vegetation cover were estimated.

Habitat A -  Slow backwater vegetation bed
Substrate: 50% silt, 50% sand
Vegetation: Dead tamarisk, grasses
Diversity: 0.95

Habitat B - Riffle
Substrate: 10% silt, 10% sand, 70% cobble, 10% boulder
Vegetation: ~5% Cladophora cover, detritus on streambed
Diversity: 1.2

Comments: Big Island (otherwise known as Bug Island) was home to numerous adult mosquitoes and biting flies in addition to the aquatic larvae that we identified in our samples.

BIRDS, MAMMALS, REPTILES, AND AMPHIBIANS 

We surveyed Big Island for an hour in the mid-afternoon during a slight breeze with between 25 to 50% cloud cover. The island mainly consisted of shrubs and cottonwood, with some interspersed tamarisk. There were many medium- to large-sized dead tree branches on the ground. Birds and mammals were identified by visual sightings.

During our survey, we observed the Wilson’s warbler and various other songbirds using the cottonwoods. Due to the wind and the shifting nature of the cottonwood leaves, most songbirds were unidentifiable at the time, so we conducted a follow-up survey the next morning in an attempt to document the high avian diversity present in the cottonwood trees at this location. In terms of mammals, cottontails and voles were present in the shrubs and dead branches of this island. A fellow colleague, Carson Jeffres, awoke in the middle of the night to a small rodent crawling on his face that was vole-sized but not identified.

Afternoon Survey

Morning Survey