WSP03817

• 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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sediment sorting on the recession. Consequently, deposition can occur in pool areas,
sediment may deposit in the middle of the channel, and a low flow channel may not fully
develop.

6.3. Historic Changes in Upper Arkansas River Morphology

On the Arkansas River project reach, both sediment load and hydrologic regime have
changed over the past 150 years, although not necessarily during similar time frames or
in relative magnitudes. Sediment loads have changed as a result of the California Gulch
mine tailings contributions, and potentially due to upstream tributary erosion caused by
flow augmentations. The caliber of sediment that has entered the system from California
Gulch is primarily fine-grained, although some coarse placered material may have been
contributed as well. Actual historic, existing, and predicted sediment loads were not
available for this study; it is assumed that the implementation of sediment control
measures on California Gulch has reduced the sediment inputs relative to the early 20th
century. The hydrology of the project reach has historically been affected by flow
augmentation, although historic augmentations are not reflective of current management
practices (BOR, 1999). Additional human impacts to the river morphology include land
use practices, specifically livestock grazing, as well as the impact of the tailings toxicity
on riparian vigor and associated bank stability.

To determine the geomorphic evolution of the project reach, aerial photographs dated
1939,1957, and 1996 were obtained and evaluated in terms of plan form dynamics and
trends in channel width. Flow duration curves were utilized to estimate the frequency of
occurrence of hydrologic events that would drive significant channel adjustments.

6.3.1. Sinuosity and Split Flow

Channel planform characteristics are those features of a channel as seen in map view.
Channel sinuosity is a measure of stream length versus down valley length, and is a
primary planform descriptor. Split flow conditions refer to the presence of multiple
channel threads under low flow conditions; this includes locally braided reaches where
extensive bar deposits are present, to long segments of multiple, largely separate
channels. Channel sinuosity and split flow as determined from aerial photographs dated
1939, 1957, and 1996 depict significant changes along the project reach (Tables 6.1 and
6.2; Figures 6.1 and 6.2). In general, sinuosity decreases in the downstream direction
through the project reach, from a high of 1.25 upstream of California Gulch to a low of
1.08 near Balltown.

Subreach I, upstream of California Gulch, experienced an increase in sinuosity from
1939 to 1957 and a subsequent decrease from 1957-1996 (Table 6.1; Figure 6.1). This
trend reflects the natural dynamics of a multi-channeled system; from 1939 to 1957, the
primary channel became more sinuous, which reduced its hydraulic efficiency. In 1992,
that channel avulsed into another channel, forming a more efficient, less sinuous thread.
This reach is upstream of the California Gulch sediment source. The 1992 relocation may
have been in part driven by flow augmentations, although the process occurs naturally in
multi-channeled systems.

May 7, 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

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