Cosmogenic 26Al, 10Be, 14C, and 36Cl dating of fluvial fill terraces and other geomorphic surfaces along steep canyons of the Colorado Front Range provides a temporal framework for analyzing fluvial response to sediment budget perturbations. Fluvial terraces > 4 m above the Middle Boulder Creek channel (grade) record activity during late Pleistocene time. Terraces < 4 m above grade and alluvial fans preserve evidence for system responses to Holocene climate change. The fluvial processes that these surfaces record contributes to a comprehensive understanding of the long-term evolution of Front Range canyons.
Cosmogenic exposure ages of boulders on glacial moraines near Boulder Canyon are consistent with glacial chronologies derived from nearby regions. Samples from Bull Lake moraines have minimum average 10Be and 26Al ages of 101 ± 21 ka and 122 ± 26 ka, comparable to other age estimates of 150 to 100 ka for moraines at sites in Wyoming, southwest Montana, and other regions of Colorado. Pinedale moraines near Nederland have average model 10Be and 26Al ages of 16.9 ± 3.5 and 17.5 ± 3.6 ka, consistent with other cosmogenic exposure ages and 14C estimates of 35 to 15 ka reported elsewhere in the Rocky Mountains. These new age estimates, considered together with field relationships, suggest that late Pinedale glaciers were nearly as extensive as those active during earlier Pinedale and Bull Lake advances.
The fluvial response to glaciation in the Front Range can be analyzed using regional and local late Pleistocene glacial chronology and cosmogenic exposure age estimates for fill terraces in Boulder Canyon. Results from Boulder Canyon show that terrace heights above grade can be divided into (1) Bull Lake (> ~100 ka) at 15 to 20 m above grade; (2) Pinedale (30 to10 ka) at 4 to15 m above grade; and (3) Holocene age at < 4 m above grade. Better-preserved terraces of latest Pleistocene age suggest that variations of height above grade reflect short-term fluctuations in the river profile during periods of rapidly changing stream load and power. Net river incision apparently occurred during transitions to interglacial periods. Soil development and stratigraphic position, used with limited cosmogenic and 14C dating suggests that ~ 130 ka terraces in Boulder Canyon correlate with the Louviers or Slocum Alluviums , and that 32 to 10 ka fills in the Canyon correlate with the Broadway Alluvium on the High Plains east of the Front Range. Middle to late Pleistocene incision rates represented by high terraces in Boulder Canyon are higher than early Pleistocene rates, possibly as a result of east to west migration of a knickpoint on Middle Boulder Creek.
Low terraces (< 4 m above grade) and alluvial fans in Boulder Canyon record Holocene deposition resulting from forest fires related to climate change. Forest fires destabilized small (0.5-1.0 km2) tributary catchments within the canyon during the early and late Holocene, producing episodes of alluvial deposition. Alluvial fans accumulated at rates between 0.2 and 95 Mgha-1yr-1 depending on whether we assume constant or episodic sediment accumulation rates. Alluvial deposition and associated forest fire occurrence in Boulder Canyon correlate with a late Holocene climate characterized by conditions drier than at present. Fire and deposition is also correlated with the warm and wet early Holocene climate which likely induced widespread vegetation growth to fuel extensive fires during occasional dry periods. Alluvial fans and fluvial terraces appear to be effective in preserving evidence of past climate change, and of processes that have contributed to the evolution of the Colorado Front Range.