Pilgrim Creek near Moran, Wyoming
Pilgrim Creek flows from a 50-mi2 drainage of the Teton Wilderness Area near Moran, Wyoming. Although Pilgrim Creek has no long term stream gage, the annual mean flow was estimated to be 52 ft3/s with a mean annual runoff of 14 inches. The highest peak flow recorded was 2,250 ft3/s on June 5, 1997, which was very close to the 100-year flood peak. The lowest mean-monthly flows, 8.8 ft3/s, occur during January and February (U.S. Department of the Interior, 1998). Pilgrim Creek has large sediment loads of sand, gravel, and cobbles. The creek flows across an alluvial fan within Grand Teton National Park before entering Jackson Lake on the Snake River. Jackson Lake is a naturally deep lake (maximum depth of 400 feet) that was raised 40 feet by the construction of Jackson Lake Dam. Originally, a timber-crib dam was privately constructed during 1906-1908, but that dam failed soon after its construction in 1910. The left abutment of the dam had been built on the Pilgrim Creek alluvial fan near the active channel of Pilgrim Creek. Before a new dam could be constructed, the downstream most portion of the creek had to be moved so that it entered the lake farther upstream from the dam site. Reclamation rebuilt a larger dam at this location, which was completed in 1916 (Piety and others, 1998).
Ever since the dam's reconstruction, Reclamation has been managing Pilgrim Creek to stay away from Jackson Lake Dam and to enter the reservoir rather than the Snake River channel downstream. Management activities of the creek have included the excavation of the river gravels to construct channels and dikes across the alluvial fan. However, in 1996, the granting of a permit for channel maintenance activities became a controversial issue. The U.S. Army Corps of Engineers was not willing to grant a permit to construct dikes with river alluvium. However, an emergency permit was granted under the condition that Reclamation fully study the continuing need to build dikes and channelize the creek (Piety and others, 1998). The key questions for this study included:
- Is the river still aggrading across the alluvial fan?
- What is the risk of the creek causing erosion of Jackson Lake Dam?
- What is the risk of the creek entering the Snake River downstream from Jackson Lake Dam?
The study concluded that the creek was transporting its sediment load through the upper reach of the alluvial fan. However, coarse sediments had been aggrading the middle reach of the alluvial fan and causing channel avulsions. Geomorphic analysis including radiocarbon 14C dating determined that the Pilgrim Creek channel had not changed its course across the upper alluvial fan during the past 2000 years. Analysis of historic aerial photographs revealed that the creek channel was wide, straight, and braided through the upper fan reach and that channel avulsions had occurred in the middle reach of the alluvial fan during the last several decades. After the coarse sediments had deposited in the middle fan reach, the creek channel was narrow and meandering in the lower fan reach.
Cross-section surveys of the creek channel revealed that the longitudinal slope decreased in the middle fan reach. Sediment load measurements and transport calculations indicated that the sediment transport capacity was relatively high through the upper fan reach, but that it decreased through the middle and lower fan reaches. Analysis of sediment samples from the creek bed proved that the coarse gravel and cobble-sized sediments were not being transported all the way through the lower fan reach.
During the 1980's, Reclamation had excavated a straight pilot channel through the middle fan reach of the alluvial fan. During the course of the study, Pilgrim Creek flows were unusually high from the 1997 spring snowmelt. Hydrologic analysis of the discharge data indicated that the maximum flow that year was near the magnitude of the 100-year flood peak. The unusually high flows from the 1997 spring snowmelt caused the new pilot channel to erode and become wider and steeper through the middle fan reach. The high flows also caused the channel through the lower fan reach to erode its banks and become wider and straighter. The straightening of the channel also made it steeper and gravels were transported through the entire middle fan reach and through most of the lower fan reach for the first time. The observations and measurements that were made during the high flows of 1997 were valuable for understanding the physical processes of Pilgrim Creek and for making future predictions. The risk of Pilgrim Creek having a channel avulsion and causing damage to Jackson Lake Dam or entering the Snake River downstream from the dam was judged to be very low. The Pilgrim Creek Maintenance Program had ultimately been successful in forcing Pilgrim Creek into a stable channel that now enters Jackson Lake much farther upstream than the channel location of the early 1900's.
For the aggrading channel of Pilgrim Creek, the field measurements of bed-material surface size, radiocarbon dating of terrace soils, and channel topography were all very useful. Survey data of the Snake River channel, downstream from Pilgrim Creek, were never used. The analysis of aerial photographs, river hydraulics, and sediment transport capacity were necessary to support the study conclusions.