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Cache la Poudre River Drainage Basin

The Cache la Poudre River drains east from its headwaters in Rocky Mountain National Park, at 3278 m elevation, to join the South Platte River near Greeley at 1400 m elevation. The Poudre River drains 4895 km2 of the Colorado Front Range and adjacent plains, and is one of the principal mountain tributaries of the South Platte River. Longitudinal variation in valley and channel morphology along the Poudre, and in process domains, can be used to represent the other major mountain tributaries of the South Platte; the Big Thompson River, Boulder Creek, St. Vrain Creek, Boulder Creek, Clear Creek, Tarryall Creek, and the headwaters of the South Platte.

This map of the Poudre River drainage basin illustrates the spatial distribution of the 8 basic valley-bottom process domains that are described in this website. Two of these process domains (colluvial hollows, ephemeral channels) are located in the headwaters of the drainage basin. The remaining 6 process domains are on the main channel, which we arbitrarily defined as starting with a minimum drainage area of 100 km2. The remainder of the channel network, designated here as “other process domains,” is composed of intermediate-scale channels for which we have not yet defined process domains. We will be adding process domains for these channels within the next year.

We have chosen to use the Poudre River drainage for our initial mapping of process domains in the Front Range because we are most familiar with this drainage. Over the next few months, we will be adding similar maps for the Big Thompson, St. Vrain, Clear, and upper South Platte drainages.

Watershed Description Map Explanation Map of Water Supply and Storage in Watershed Poudre Photo Gallery (Includes Maps and Photos)

Description of Cache La Poudre River Watershed

The mountain portion of the Poudre River drains 2750 km2. Tributary channels begin at 3280 m elevation, and the mainstem flows beyond the mountain front to its junction with the South Platte River in Greeley, Colorado. For its first 20 km, the Poudre flows through a broad, low-gradient valley before entering a canyon approximately 95 km long, in which the river remains until it flows beyond the mountain front. The terminal Pleistocene-age glacial moraine is located 55 km downstream from the headwaters. Much of the mountainous portion of the Poudre drainage lies within Roosevelt National Forest, and some of the headwaters are within Rocky Mountain National Park. The upper reaches are also Colorado’s only designated Wild and Scenic River. State highway 14 parallels much of the river’s course within the mountains, and Fort Collins (population 135,000) is the largest city within the drainage. The non-governmental organization American Rivers listed the Poudre River at #3 in its 2008 list of America’s most endangered rivers because of proposed flow diversions along the lower Poudre River.

Picture of glacial unconfined reach in Pingree Park

Pingree Park

(Photo by E. Carlson)

Picture of glacial unconfined reach in Rocky Mountain National Park

Glacial unconfined reach in Rocky Mountain National Park

(Photo by E. Carlson)

Picture of Poudre in Pingree Park

Pingree Park

(Photo by E. Carlson)

Info and pictures for water supply and storage map

Map of Cache la Poudre River Basin

(Map created by E. Carlson, 2008)

Map Explanation

Classification of process domains in the Poudre River watershed was envisioned as a way to simplify complex fluvial systems and group streams with similar geomorphic characteristics. Experience in fluvial geomorphology and knowledge of the watershed by contributing members were used to develop a classification of the main channels in the watershed. The entirety of the watershed was classified with segments of channel and surrounding areas described by one of eight classes.

The definition of our main stem rivers was those which drained areas greater than 100 km2. This threshold was chosen through previous field reconnaissance and restricted by the goals of the project. The Big South branch of the Poudre River was compared to similar drainages in other Front Range watersheds and it was determined that a significant shift in process was likely to occur at the 100 km2 mark. One of the goals of the project was to simplify the variability inherent in river systems. Completing the classification at a coarse scale with a minimum mapping unit (MMU) of 1000 meters helped satisfy this goal.

Data Input

The basic data input used was a digital elevation model (DEM) with a horizontal resolution of 10 meters covering the entire Poudre River watershed, extending past the mountain front. From this data layer calculations of valley slope, hillslope, and valley width were performed to aid in developing the classification. The degree of confinement was used as the primary characteristic with which to categorize the land and streams. Confinement was defined as valley bottom width. The boundary between the valley bottom and the hillslope was defined using flood information from the Front Range to determine a maximum depth of inundation for a flood magnitude of greater than 100 years. A range from three to five meters was obtained and used to assess the width of the valley bottom. Using cross-sections measured remotely in Arc GIS 9.2, we were able to break the spread of width measurements into three classes of confinement. The spread of cross-section widths was combined with knowledge of the river system to develop the width classes.

Map Creation

The maps were created by "hand" meaning that each segment of main stem river (between minor tributary junctions) was examined. The MMU of 1000 m was a logical choice for this type of work. Fluvial processes perform work on a range of spatial scales. The scale the project decided to address was towards the coarser end of spatial resolution. Variability in contiguous river segments that totaled less than 500 m were combined into the dominant process type of the river segments. As the drainage area falls below 100 km2 it was expected that the fluvial processes would begin to act on a different spatial scale than this project was addressing. Therefore much variability could be present in areas classified as Other Processes, but were not addressed in this project. The areas designated Colluvial Processes are present in the uplands sections of the watershed following ridgelines between the channel networks. This process domain could play an important role in influencing other process domains, likely the Confined domains, where hillslope activity could be directly coupled to fluvial processes.

Water Supply and Storage in Cache La Poudre Basin

Simplified map of water diversions and storage in Poudre Basin

Map showing main canal and ditches with direction of flow towards the Poudre Basin. Click below for pdf of full map.

(Map prepared by Chris Sturm, Colorado Water Conservation Board)

Below is a link to the pdf of water supply and storage map compiled by Chris Sturm:

diversion pdf

 


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