Sediment Delivery by the Yukon River to the Yukon Flats, Yukon Delta and the Bering Sea

The physical, chemical and biological attributes of the Yukon River and tributary basins impact soil erosion, sediment transport and sediment delivery. The glacier, snow and permafrost melting, runoff, erosion, transport, deposition and storage of gravelly, sandy, silty and clayey sediments determine the habitat distribution and water quality within the river channels and floodplains. The ecological functioning, with food and nutrient delivery, migratory cues, breeding, habitats, and riparian and floodplain ecological cycles are all dependent on the transported sediment at specific times of the year. Annual temperatures have been rising since the 1840s which could contribute to higher runoff water flows and greater sedimentation. The primary objective was to document the sedimentation in the Yukon watershed with little soil erosion as a result of agriculture or urban development. The causes of the soil erosion and sedimentation were permafrost, alpine glacial melting, drilling for gas and oil, road construction, gold mining, cold war military sites, pipeline construction, forest fires and steep slopes.


Introduction
The Yukon River (Figure 1   have a subsistence lifestyle depending on edible plants and roots, berries, fish and game. The climate topography is quite variable [1]. Wetlands account for 30% of the land use. Annual temperatures have been rising since the 1840s which could contribute to higher runoff water flows and greater sedimentation [1]. Melting glaciers ( Figure 6) add 29% to the flow in the Tanana and White    cumulative effects of man's activities on the Yukon River Basin cannot be made due to limited availability of water quality data [2]. With hotter and dryer conditions there has been an increase in forest fires and melting of the permafrost ( Figure 9) which leads to increased water flows, sediment transport ( Figure 10) and organic carbon transfer to Yukon River bottomlands, Yukon Flats, Yukon Delta ( Figure 11) and the Bering Sea [1]. The primary objective was to document the sedimentation in the Yukon watershed with little accelerated soil ero-

Alaska Geological History
The Cordilleran Ice Sheet was a major ice sheet that periodically covered much   . Wall of streambank sliding into a river with permafrost and peat exposed. of North America during the last 2.6 million years. The glaciers in the Yukon River Basin were mostly alpine glaciers ( Figure 13) rather than continental glaciers due to insufficient moisture [2].       dry herbaceous, ice/snow and rivers, streams and lakes.

Exploration of the Yukon River Basin
The

Yukon Flats
The Yukon Flats are centered on the confluence of the Yukon River, Chandalar A few thousand Alaska Natives and others live in the Yukon Flats watershed within a few small villages and seasonal settlements including hunting cabins [2]. The region contains large deposits of crude oil and natural gas. This has led to a conflict between protecting wildlife and drilling interests. A proposed land trade was made in 2008 between private sector land owners and the federal government but it did not happen; however, trade talks are still ongoing.

People and Land
In the Canadian part of the Yukon River Basin, Whitehorse is the center of population with just over 23,000 residents in 1998 [4]. The town of Dawson City Atlin Provincial Park is located near the headwaters of the Yukon River. These lands compose about 9 percent of the land area of the Canadian Yukon.

Yukon-Kuskokvim Delta
The Yukon-Kuskokwim Delta is where Yukon and Kuskokwim rivers flow into the Bering Sea. The delta is 129,500 km 2 and located on the west coast of Alaska [1]. It is larger than the Mississippi River Delta. The delta consists of tundra (and has approximately 25,000 residents). Eighty five percent are Alaska Natives      be trucked to Circle, stored outside at a trading post on the banks of the Yukon River ( Figure 25) and then loaded on boats or barges for the downriver journey to Fort Yukon ( Figure 26).

Soils of the Yukon River Basin
In the Yukon River Basin, the type of parent material, climate and relief have

Peatlands
Peatlands are also expected to be impacted by natural global climate change.
Peat is made up of decomposing organic material, and so is very rich in carbon.
It consists of 90% water and 10% plant matter, and is mostly found at the high latitudes of the northern hemisphere, both at the surface and below. Some of this peat is found underneath the permafrost layer (Figure 9), which means the carbon it harbors could be released to the atmosphere by microbes the permafrost should melt.

Permafrost
Permafrost is soil, rock or sediment that is frozen for more than two consecutive years. In areas not overlain by ice ( Figure 29), it exists beneath a layer of soil, rock or sediment, which freezes and thaws annually and is called the "active layer." In reality, this means that permafrost occurs at a mean annual temperature of −2˚C or colder. Active layer thickness varies seasonally. The extent of permafrost varies with the climate: in the Northern Hemisphere today, 24% of the ice-free land area, equivalent to 19 million km 2 [7], is more or less influenced by permafrost. Of this area slightly more than half is underlain by continuous permafrost, around 20 percent by discontinuous permafrost, and a little less than 30 percent by sporadic permafrost [8].

Seasonal Melting of the Permafrost
During summer in the Arctic, the soils warm fast and frozen soils start to thaw ( Figure 30). When the ice layer melts, the soil organic carbon-rich soil oozes Open Journal of Soil Science from permafrost layer. As the temperature of the ground rises above freezing, microorganisms break down organic matter in the soil [9]. Greenhouse gases, including carbon dioxide, methane and nitrous oxide, are released into the atmosphere. Soils in the permafrost region hold twice as much carbon as the atmosphere does-almost 1600 billion mt [10]. Some of the soil organic matter is decomposed by the microorganisms and carbon dioxide is released into the atmosphere.

Carbon Cycle in Permafrost
The permafrost carbon cycle deals with the transfer of carbon from permafrost soils to terrestrial vegetation ( Figure 31)

Methane
In moist areas, most of the emissions will be of methane, a greenhouse gas that has 20 to 25 times more warming power than carbon dioxide. As the ground warms, methane will either be released directly into the atmosphere or bacteria will break it down into carbon dioxide, which will then be released. If areas of thawed permafrost exist at depth between frozen layers (Figure 30), it is possible that microbial activities will continue unabated, even during the winter, to create  new methane from organic material.

Natural Climate Change Effects
Arctic permafrost has been diminishing for many centuries. At the last Glacial Maximum, continuous permafrost covered a much greater area than it does today. The consequence is thawing soil, which may be weaker, and release of methane, which contributes to an increased rate of global warming as part of a feedback loop.
The ground can consist of many substrate materials, including bedrock, sediment, organic matter, water or ice. Frozen ground is below the freezing point of water, whether or not water is present in the substrate. Ground ice is not always present, as may be the case with nonporous bedrock. By definition, permafrost is ground that remains frozen for two or more years. Since frozen soil, including permafrost, comprises a large percentage of substrate materials other than ice, it thaws rather than melts. One visible sign of permafrost degradation is the random displacement of trees from the vertical in permafrost areas [12] [13].

Alyeska Pipeline
The Trans-Alaska Pipeline System (TAPS) the trans-Alaska crude-oil pipeline

Construction on Permafrost
Building on permafrost is difficult because the heat from its construction along the pipeline or paved road can thaw the permafrost and destabilize the structure.
Three common solutions include: using foundations on wood piles; building on a thick gravel pad (usually 1 -2 meters); or using anhydrous ammonia heat pipes [14]. The Trans-Alaska Pipeline System (Figure 34) uses heat pipes built into

Effect on Slope Stability
Over the past century, an increasing number of alpine rock slope failure events in mountain ranges around the world have been recorded. It is expected that the high number of structural failures is due to permafrost thawing ( Figure 27), which is thought to be linked to climate change [1] [10] [15]. In mountain ranges, much of the structural stability can be attributed to glaciers ( Figure 13) and permafrost. As climate warms, permafrost thaws, which results in a less stable mountain structure, and ultimately more slope failures [15]. Increasing temperatures allow deeper active layer depths (Figure 9), resulting in increased water infiltration. Ice within the soil melts, causing loss of soil strength, accelerated movement, and potential debris flows [16].
Instability of slopes in permafrost at elevated temperatures near freezing point in warming permafrost is related to effective stress and buildup of pore-water pressure in these soils. McSaveney [17] reported massive rock and ice falls ( Figure 6), earthquakes, floods, and rapid rock-ice flow to long distances caused by "instability of slopes" in high mountain permafrost.

Ecological Consequences
In the northern circumpolar region, permafrost ( Figure 27) contains 1760 billion mt of organic material equaling almost half of all organic material in all soils [7]. This pool was built up over thousands of years and is only slowly degraded under the cold conditions in the Arctic. The amount of carbon sequestered in permafrost is four times the carbon that has been released to the atmosphere due to presumed human activities in modern time [10]. One manifestation of this is yedoma, which is an organic-rich (about 2% carbon by mass) Pleistocene-age loess permafrost with an ice content of 50% -90% by volume [18]. Open Journal of Soil Science Formation of permafrost has significant consequences for ecological systems, primarily due to constraints imposed upon rooting zones, but also due to limitations on den and burrow geometries for fauna requiring subsurface homes. Secondary effects impact species dependent on plants and animals whose habitat is constrained by the permafrost. The dominance of black spruce in extensive permafrost areas occurs since this species can tolerate rooting pattern constrained to the near surface [19].
The Arctic region is one of the many natural sources of greenhouse gas methane [20]. Global warming accelerates its release, due to both release of methane, from existing stores and from methanogenesis in rotting biomass [18].
Large quantities of methane are stored in the Arctic in natural gas deposits, permafrost, and as submarine clathrates (host-guest complexes). Permafrost and clathrates degrade on warming, thus large releases of methane from these sources may arise as a result of global warming [21]. Other sources of methane include submarine talks (year around unfrozen soil layers), river transport ( Figure 42 and Figure 30), ice complex retreat, submarine permafrost and decaying gas hydrate deposits [22].

Predicted Rate of Change in the Arctic
According to Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report [23], there is high confidence that permafrost temperatures have increased in most regions since the early 1980s. Observed warming was up to 3˚C in parts of Northern Alaska (early 1980s to mid-2000s) [24]. In the Yu-

The Effect of Natural Climate Change on Permafrost
The upper layer of permafrost, or the active layer, sometimes thaws in the summer. Climate change is expected to significantly affect above and below-ground climate [1] [10] [15]. Recent studies have shown that there has been a decrease in freezing during the cold season in North America's permafrost regions.
Coastal areas and eastern Canada have started to see significant increases in  Another natural factor that can impact permafrost is fire. Wildfires [28] disturb thousands of hectares of land in the Yukon River Basin each year. The wild fires expose soil to erosion, transport and deposition. Foot [28] has estimated that nat-

Anthropogenic Effects on Water Quality
Discussions of the water quality of the Yukon River Basin are based on limited data and indicate that water chemistry differences throughout the basin are due more to natural factors rather than to human-induced factors. However, the basin has been affected by limited human activities within the basin and from outside the basin. The difficulty arises in determining to what degree humans have affected the water quality, because a suitable water-quality data base does not exist at the present time. The Yukon River Basin is not subject to the intensive agricultural cultivation or application of organic pollutants found in some rivers of the lower 48 states. It is more vulnerable to global atmospheric transport of pollutants that is well recognized.
In the northern hemisphere, transport occurs primarily in the winter months when temperature and pressure gradients are the greatest. Pollutants from mid-latitudes are transported northward, where greater precipitation and colder temperatures cause deposition from a "warm-cold distillation" effect [29]. Chlo- DDT, and PCB's were found in burbot (Lota lota) liver, and lake trout (Salvelinus namaycush) and whitefish (Coregonus clupeaformis) muscle in Lake Laberge near the headwaters of the Yukon River [30]. The concentrations in Lake Laberge whitefish were 3 to 42 times higher than those in whitefish from other lakes in the region [30]. Atmospheric transport was determined to be the source of the pollutants in predatory fish [31].
Mining activity (Figure 8) has, and continues to be, an important economic industry in the Yukon River Basin. Probably the biggest concern of mining is the possible harm to fish-spawning areas. Although today's mining practices are highly regulated to prevent damage to fish habitat, many old abandoned mine areas remain to be reclaimed. The effect on water quality has yet to be determined.   In areas of the Yukon River Basin with discontinuous permafrost, the riverbanks may be permanently frozen and overlain by seasonally frozen layers of organic material and plants. This condition creates an additional source of sediments in the summer when permafrost melts while flowing water transports sediment into the streams and rivers. Most of the measured suspended sediment concentrations for the mainstream of the Yukon River were less than 1000 mg/l.

Sediment Sources
The two major glacier-fed rivers, the White and the Tanana had the highest concentrations.

Summary and Conclusions
The