Boulder Creek CZO sites Map
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The Boulder Creek Watershed
The Boulder Creek Critical Zone Observatory (CZO), located in the Front Range of Colorado, is designed to examine the effect of erosion on the development and function of the Critical Zone. The Critical Zone is the near-surface region that supports terrestrial life, extending vertically from the base of groundwater to the upper reaches of the vegetation canopy. The Boulder Creek Watershed is about 1160km2 and drains the Front Range from the Continental Divide (4120m) to the eastern plains (1480) (Murphy, 2000). In the upper reaches of Boulder Creek, it has 3 tributaries – North Boulder Creek, Middle Boulder Creek and South Boulder Creek. The confluence of North Boulder Creek and Middle Boulder Creek is at ?m. South Boulder Creek joins Boulder Creek near the City of Boulder.
The large elevation range within the watershed encompasses five climatic zones – alpine, subalpine, montane, foothills and plains. Total annual precipitation generally decreases with elevation, from >100cm at the Continental Divide to about 46cm near the City of Boulder. The project will focus on the upper basin of the Boulder Creek Watershed (above the City of Boulder).
Three Distinct Erosion Regimes
The upper half of the watershed is a mountainous landscape of crystalline rocks, while the lower half crosses the piedmont and is underlain by sedimentary rocks and lined with alluvial terraces. The project will focus on the mountain portion, carved in Precambrian (1.7 by) granodiorite and older biotite gneiss (Lovering & Goddard, 1950), where three distinct erosion regimes arise (Birkeland et al., 2003).
At the crest, the watershed was glaciated. U-shaped valleys, cirques (a few still with ice), and rock-dominated valley floors are found. Cosmogenic radionuclide (CRN) exposure ages on glacially polished bedrock in the valley floor document that glacial erosion removed the previous cosmogenic production layer, and that glaciers retreated from their maximum position at ~17ka to the divide between 14-12 ka (Ward et al., 2006). East of the glacial limit is a broad, high (2500-2750 m) post-Laramide surface of low relief (Bradley, 1987), in which weathered rock profiles are up to 15 m thick (Isherwood & Street, 1976; Dethier & Lazarus, 2005). Still further east, renewed bedrock channel incision over the last 5 My has progressed headward from the plains, cutting deeply into the post-Laramide surface, forming the steep slopes and deep canyons of the range front (Anderson et al., 2006).
The glacially scoured headwaters, the post-Laramide low relief surface, and the deeply-incised range front canyons are all in close proximity and are developed in similar granitoid rocks. The variation in weathered profile development resulting from these erosion regimes therefore constitutes a natural experiment that will enable us to test coupled models of weathering front advance, regolith production and sediment transport in an accessible field laboratory. The impacts of this spatial variation in the critical zone on hydrologic and biologic activities can also be documented, which will in turn constrain hydrologic and ecological models.
Geology of the Boulder Creek Watershed
The last episode of mountain building to shape this landscape started roughly 65 million years ago in the Laramide Orogeny. This brought hard crystalline rock from great depths, and juxtaposed it with softer Mesozoic and Paleozoic sedimentary rocks. This transition in rock type still dominates by localizing the abrupt mountain front. Decay of the mountains since the end of the Laramide Orogeny about 40 million years ago resulted in slow erosion and smoothing of the mountains, and construction of a ramp of sediment sloping down to the east across the plains. Perhaps as recently as 7-3 million years ago, the ramp of the Great Plains was disrupted and incised on its upstream, western edge by the South Platte River and its tributaries. This incision acted as a baselevel fall for the streams draining the harder rocks of the crystalline core of the Front Range, and has sent a wave of incision up each of these rivers. The range is therefore in the midst of a transient response to the incision of the western edge of the Great Plains. In the same timeframe, in the last couple million years, or Pleistocene time, alpine glaciers carved U-shaped valleys and cirques into the headwaters of these river basins.
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