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3. HISTORICAL PERSPECTIVE 3.1 Geomorphologic History 3.1.1 AntiQuitv-1800 The ancestral Arkansas River probably cut its first river valley during the earliest known geomorphic cycle in Quaternary time. Analysis of terrace gravel deposits reveals that this initial geomorphic cycle was followed by five more geomorphic cycles during the Pleistocene, and in each geomorphic cycle, the Arkansas River cut a deeper valley and deposited gravel on its floor. During this downcutting, the Arkansas River also moved laterally. Geologic evidence indicates that the river between Fowler and La Junta, Colorado, has migrated northward as much as seven miles, and between La Junta and Kansas it has migrated southward as much as nine miles (3). Migration of the river northward or southward across its valley probably resulted from a greater discharge of sediment into the river channel from one side of the valley than the other. Tributaries that discharge large volumes of sediment build alluvial fans at their junctions with the river. Unable to remove the sediment as quickly as it is deposited, the river flows around the edges of the fans. In so doing, the river cuts into the opposite wall of the valley and gradually moves away from the growing fan. Along the river between La Junta and Kansas, for example, the tributaries from lhe north are long and flow over unconsolidated, erodible deposits. The tributaries from the south are shorter and flow over bedrock, which is less erodible. Due to these physical differences, the northern tributaries supplied a greater volume of sediment to the river than did the southern tributaries, and the river migrated southward (3). 3.1.2 l800-Present Measurements and reports by explorers in the early 1800' s indicate that the Arkansas River was relatively straight, wide, shallow, braided, and intennittent, with sparse bank and floodplain vegetation (2). The River has changed dramatically during the past 150 years. Today it is narrower and more sinuous due to perennial stream flow. Large meander loops are evident, and there is a substantial increase in bank and floodplain vegetation, especially in the phreatophyte salt-cedar. 3.2 Economic History 3.2.1 19th Century Development of the upper Arkansas River basin has progressed through several stages, beginning with the early 19th Century era of fur trappers and mountain men, who explored the region in search of fur-bearing animals, and traders who established pioneer posts at points along the base of the foothills. One of the first of these mercantile and social centers was built in 1832 by the Bent brothers, near the present site of La Junta. Ten years later James P. Beckworth established a trading post and began settlement of 6