WSP02347

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 5/7/1999
• Author: Inter-Fluve Inc
• Title: Fluvial Geomorphic Assessment of Upper Arkansas River - Final Report
• Water Supply Pro - Doc Type: Report/Study

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4. Physical Characteristics of the Upper Arkansas River
Basin

4.1. Geologic Setting

The upper Arkansas River basin consists of a long structural downfaulted trough that
represents the northern terminus of the Rio Grande Rift (Karnuta, 1995). Primary
tributaries to the upper Arkansas River drain eastward out of glacial valleys that dissect
the Sawatch Range. Glaciers occupied the major western and northeastern valleys
entering the Arkansas River Valley, Although the Arkansas Valley itself was not
glaciated, individual ice tongues extended across and locally blocked the valley (Chronic,
1980).

Within the project reach, the Arkansas River flows through a valley fill composed of
poorly sorted, unconsolidated sediments that are late Pleistocene and Holocene in age and
glacial and fluvial in origin. These units form relatively old high terraces, as well as the
existing channel perimeter and active floodplain. The most extensive and highest terrace
is approximately 360 ft above the river, and extends from Malta to Big Union Gulch
(Parker, 1994). This terrace is prominently exposed on the western valley margin, and
forms a steep valley wall against the active floodplain.

Downstream of the Highway 24 bridge, the river flows along other geologic mapping
units as it flows along the eastern valley margin towards Balltown. Near Mt. Massive
Lakes, the river flows along the toe of a unit mapped as having formed by mass
movement events, such as landslides, talus accumulations, or rock glaciers (Collins and
Collins, 1986), Tertiary-age volcanic and sedimentary units are exposed along the
eastern valley wall from Mt. Massive Lakes to downstream of Kobe. From Kobe
downstream to Balltown, the channel flows against the eastern valley margin, which is
composed of Precambrian granites (Collins and Collins, 1986).

4.2. General Geomorphic Characteristics of Study Reach

Six primary geomorphic subreaches (Subreaches 2-7) were delineated for the II-mile
project reach (Tables 4.1 and 4,2, Figure 1.2). A seventh subreach (Subreach I) extends
upstream of California Gulch. These individual subreaches can be described in terms of
distinct geomorphic characteristics and processes. General physical characteristics of
each subreach are described below. Plotted profiles for each subreach are included in
Appendix A.

4.2.1. Subreach 1: Upstream of California Gulch (RM 10.9-12)

Subreach I is located upstream of the main project reach, extending from an old road
crossing downstream approximately I valley mile to the California Gulch confluence
(Table 4.1; Figure 1.1), The reach has not been impacted by tailings delivered to the
river by California Gulch, although it does experience substantial flow augmentation, and
some mines are located within its watershed. However, the extensive tailings deposits

May 7. 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

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