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periods in summer and fall. Flashy runoff and erosion in small <br />tributary streams are promoted by poor soil development and lack <br />of vegetation. When they are flowing, Red Wash, the Chaco River, <br />and other tributaries contribute heavy loads of silt to the San <br />Juan River. <br />Five diversion structures exist within the study area <br />(Figure 3). These range from relatively inefficient soil-and- <br />boulder dikes bulldozed in the channel to concrete weir <br />structures over which the entire river flows. The first <br />diversion (La Plata) was located just upstream of the confluence <br />of the San Juan and La Plata rivers. It consisted of a boulder- <br />and-debris levee that presented no navigation problems. The <br />second diversion (Weir) was a concrete weir located just <br />downstream of Fruitland. The entire river passed over this <br />structure and necessitated portages on every trip. At extremely <br />high flows (ca. 285 m3/s), a portion of the river would pass <br />around this weir and allow upstream movement of fishes. The pump <br />station diversion (Power Plant) located three river miles <br />downstream of the weir, was also a structure over which the <br />entire river passed. It is navigable only during periods of high <br />flow (>85.7 m3/s). The Hogback diversion was a boulder embayment <br />that forced water into a canal. We were able to navigate this <br />diversion in May and August 1987, but a newly-constructed rock-- <br />soil dike forced us to portage equipment in October. The final <br />diversion at Cudei (Wind in the Canyon) was the only one located <br />downstream of Shiprock and was traversed on all three trips <br />without difficulty. Varying flow regimes over short time <br />11