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Aggradation of glacially-sourced braided rivers at Mount Rainier National Park, Washington: Summary report for 1997-2012

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Author(s): Scott R. Beason, Laura C. Walkup, Paul M. Kennard

Document Type: Natural Resource Technical Report NPS/MORA/NRTR-2014/910
Publisher: National Park Service
Published Year: 2014
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Rivers are dynamic forces of nature whose form and function are driven by sediment inputs balanced with stream flow. When sediment production overwhelms stream flow in a river, excess sediment accumulates across the river bed, in a geologic process called aggradation. If stream flow exceeds sediment production, the river incises. All rivers go through episodes of aggradation and incision in an attempt to equilibrate stream flow and sediment inputs. However, when a river is continually provided more sediment than it can transport, sediment is deposited in the active channel, floodplain size increases and threats like flooding and debris flows can be of greater consequence over time. Combined with uncertainties in future climates, geologic hazards from aggradation can increase threats to infrastructure placed near aggrading rivers.

Mount Rainier is a spectacular example of geologic forces at work - from the eruptions that built up the volcano over the last half million years, to the erosive forces that combine to tear down the mountain. In times of relative volcanic quiet, the forces of glaciers, freeze/thaw cycles, water, and wind all work to tear the mountain down. As the mountain falls apart via these forces, sediment is provided to rivers. This sediment routes through the river system in a variety of time scales. This study examines the effects of sediment movement in a 15 year period between 1997 and 2012.

Given that much of the infrastructure at Mount Rainier National Park (MORA) is built adjacent to proglacial braided rivers, it is critical to understand the rates of aggradation in these areas to anticipate the geologic future for these areas. For instance, what are the threats to roads and buildings in areas next to aggrading streams? In order to gauge these threats, we must first know the "health" of the braided river systems: are they at equilibrium, aggrading, or incising? Additionally, what do we anticipate future aggradation trends to be based on what we've observed thus far and forecasts for regional climate change in the next century?

Sediment is provided to rivers in a variety of ways, including glacial runoff, rock fall, and debris flows. The latter can provide large amounts of sediment in a very short time frame. At Mount Rainier, debris flows occur with some frequency and the park has seen at least 12 separate debris flows initiated in six drainages during events in 2001, 2003, 2005, and 2006. All of the debris flows since 2006 have initiated in areas that have been recently deglaciated, a worrisome prospect considering that retreating glaciers are continuing to expose vast areas of loose, unstable sediment on steep slopes. Given that Mount Rainier has much steep, loose terrain above 2,500 m, the potential sediment budget at Mount Rainier is very high. Additionally, some of the largest floods on record have occurred in the last two decades. The combined extremes we are seeing in the weather, hydrology, and glacial recession at Mount Rainier are consistent with models for increasing climate change in the Pacific Northwest.

In order to gauge the threats to infrastructure at MORA, cross sections were surveyed in developed locations at the park. We surveyed 27 cross sections on the Nisqually River, located on the southwest side of Mount Rainier: eight at Sunshine Point (Figure 6), ten at Longmire (Figure 8), six at Carter Falls (Figure 10) and three at Lower Van Trump Hairpin (Figure 11). Eight cross sections were surveyed on the White River, on the park's Northeastern side (Figure 12). Each cross section represents a snapshot of the geomorphic landscape at that point in time. These cross sections are re-surveyed yearly in order to identify the geomorphic landscape associated with aggradation or incision in the reach. Cross-sections are measured using a TopCon Total Station with sub-centimeter accuracy.

Sunshine Point is located just within the southwest boundary of Mount Rainier National Park, alongside the Nisqually River. The Nisqually River has received a tremendous sediment input from the South Tahoma Glacier via Tahoma Creek in the vicinity of Sunshine Point. Tahoma Creek has experienced massive sediment influx due to the occurrence of over 25 separate debris flows since 1967. This sediment input has led to high aggradation rates in the Nisqually River at Sunshine Point. Aggradation was observed in the intervals 2005-2006, 2006-2008, 2009-2011, and 2011-2012, with weighted aggradation rates ranging from 0.03 to 0.36 m*yr-1 (0.10 - 1.18 ft*yr-1). Only one year within the survey period showed net incision. The interval of 2008-2009 showed incision of -0.16 m*yr-1 (-0.52 ft*yr-1), much lower than the aggradation rates occurring before and after. Overall, the Sunshine Point reach has accumulated 26,740 m3 (944,100 ft3) of sediment from 2005 to 2012, one of the highest amounts observed in this study. Sunshine Point has been greatly affected by the Nisqually River and Tahoma Creek, especially in 2006 when a large flood destroyed the infrastructure at the campground. It should be expected that aggradation will continue to affect the Sunshine Point area due to continual sediment moving downstream in Tahoma Creek and river dredging at the Tahoma Creek Bridge (Anderson, 2013). Additionally, the riverbed in this area is "tilted" toward park infrastructure, meaning that the river will preferentially flow toward the campground remains and road. The Nisqually River here is by no means in equilibrium and it may take decades for the river to adjust to increasing sediment loads.

Longmire is home to many park maintenance facilities, visitor destinations, and employee housing. The Longmire area was also greatly affected by the 2006 flood: levees were destroyed and the park nearly lost its Emergency Operations Center. Upstream sediment delivery and sediment routing through the Longmire reach cause large variations in the sediment-to-discharge balance of the Nisqually River at this location. Aggradation and incision rates here are much more variable year-to-year compared to other locations in the park. Aggradation was observed in 2005-2006, 2009-2010, and 2011-2012 ranging from 0.05 to 0.14 m*yr-1 (0.16 - 0.46 ft*yr-1). Incision occurred in the reach between 1997-2005, 2006-2008, and 2010-2011 ranging between 0.00 to -0.04 m*yr-1 (0 - 0.13 ft*yr-1). Overall, the Longmire reach has accumulated 2,185 m3 (77,170 ft3) of sediment from 1997 to 2012. However, most areas in the Longmire reach have incised since 2006, with the notable exception of Longmire cross section 7, which has showed a rather large increase in sediment - likely relicts from dredging efforts upstream of the line. Longmire is located downstream of areas that have seen large increases in sediment delivery and as this sediment sluices downstream, it is likely that the aggradation rate here will increase in the coming years to decades, depending on timing and magnitude of large floods that move the sediment downstream.

New cross sections were added on the Nisqually River in the vicinity of the Cougar Rock Campground in 2011 and resurveyed in 2012, a location in this study referred to as Carter Falls. The Carter Falls reach is just downstream of massive sediment inputs from debris flows from Van Trump Creek and was added in an attempt to trace the downstream movement of this reworked sediment over time. In one year, the weighted aggradation in this reach was 0.10 m*yr-1 (0.33 ft*yr-1), with an influx of 2,850 m3 (100,700 ft3) of sediment between 2011 and 2012. The upper four cross sections here have strong increases in sediment volume, decreasing in magnitude downstream. The lower two cross sections both show incision; this trend is consistent with a wave of sediment moving into the survey reach from the upstream sediment deposition. Carter Falls will be a critical location to define sediment transport rates, especially its implications to downstream localities like Longmire.

The Nisqually River at the Lower Van Trump Hairpin has seen some of the most dramatic and variable changes in channel morphology since the early 2000s. This area was affected by debris flows in 2001, 2003, 2005, and 2006; as well as by numerous landslides, most notably a large landslide in 2008. Because of the large sediment delivery in this location, channel equilibrium is unlikely to be exhibited here for decades. Reach-weighted aggradation was strongly observed here in 2005-2006 as result of a 2005 debris flow that deposited an impressive 15,500 m3 (547,500 ft3) of material, an aggradation rate corresponding to 1.55 m (5.09 ft) in a one year period. Some incision has occurred in this area, namely in the intervals 2006-2008 and 2010-2011, and ranging between -0.03 to -0.18 m*yr-1 (-0.10 to -0.59 ft*yr-1). However, the periods 2008-2009, 2009-2010, and 2011-2012 have seen aggradation ranging from 0.01 to 0.12 m*yr-1 (0.03 - 0.39 ft*yr-1). Overall, 13,130 m3 (463,700 ft3) of sediment have accumulated in this area since 2005. It is anticipated that, without further sediment inputs, the Nisqually River will continually erode away at these deposits, mobilizing them downstream. However, future debris flow deposition here is very likely, so this area is not expected to return to equilibrium in the near future.

The White River is fed by the Emmons glacier on Mount Rainier, and flows 121 km (75 mi) from its source, joining the Puyallup River at Sumner. The stretch of the White River along State Route 410 on the park's northeastern side has seen rather large increases in aggradation. The riverbed here is up to 3.6 m (12 ft) above the road in some places, a floodplain disequilibrium also found elsewhere throughout the park that can have devastating consequences during high flow. Cross sections measured since 2005 have shown overall aggradation of about 0.04 m*yr-1 (0.13 ft*yr-1) during the 2005-2007 and 2008-2011 periods. Incision occurred in 2007-2008 at the rate of -0.09 m*yr-1 (-0.30 ft*yr-1). During the study period, 54,670 m3 (1,930,000 ft3) of sediment has accumulated in this reach (this reach has a much larger area than the other areas analyzed in the park). Mature old growth and forested floodplains are preventing a massive channel avulsion for now. However, aggradation of the stream channel could slowly overwhelm the stabilizing forces of the old growth forest and monitoring of this area is necessary to maintain park and state infrastructure through this study reach. Massive sediment delivery has occurred upstream as result of the Little Tahoma Peak collapse in 1963. That sediment is likely moving down-stream and may be impacting the study reach at this time.

Park-wide aggradation rates are highly variable and depend on multiple factors including location, time period, and sediment inputs. However, every location in this study has seen overall aggradation during the study, despite periods of incision. Additionally, despite the largest floods on record in the park's history occurring recently, rivers continue to aggrade, which indicates sediment delivery is overwhelming erosive forces in rivers. These results indicate that river systems at Mount Rainier are strongly driven by sediment production, a trend that we expect to remain constant or increase. Increasing aggradation rates observed at Mount Rainier are an example of the complex interactions of a glaciated landscape responding to climate change. As glacial retreat occurs in alpine areas, new unvegetated, unstable sediment is exposed and continually transported into braided rivers already choked with material. Aggrading rivers - especially those mechanically confined and not allowed to move about their natural floodplains - develop unstable convex profiles, prone to avulsion to lower-lying floodplains. Much infrastructure has been built in low-lying areas near braided rivers at MORA. As climate change occurs and as aggradation rates increase, river beds build up progressively higher, increasing flood danger to infrastructure. Flooding, damage to park infrastructure, and a record-long park closure have been attributed to the aggradation and associated avulsion occurring in Park rivers. Rivers are aggrading even without the influence of debris flows, and even during recent flood events despite heavy rain and anticipated erosive forces. It is anticipated that aggradation will have progressively detrimental consequences to areas farther away as sediment budgets increase. This is important not only to development within the park, but to the fluvial environments more distant from the park. Aggradation will present new problems to planning and engineering in glacially-sourced rivers here and in other glacial environments in the Pacific Northwest.

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Suggested Citations:
In Text Citation:
Beason and others (2014) or (Beason et al., 2014)

References Citation:
Beason, S.R., L.C. Walkup, and P.M. Kennard, 2014, Aggradation of glacially-sourced braided rivers at Mount Rainier National Park, Washington: Summary report for 1997-2012: Natural Resource Technical Report NPS/MORA/NRTR-2014/910, National Park Service,