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<br />The valley floor is the former flood plain of the Colorado River, <br />and is underlain by hundreds of feet of alluvium deposited by the <br />river. Typical composition of the alluvium is illustrated on Figure 1, <br />which contains a stratigraphic section based on a Geological Survey <br />test well in Sec. 13, T. 8 S., R. 21 E. [4]. Soils in the Palo Verde <br />Valley range in texture from fine sand to silty clay and are excellent <br />for crop production. <br />The sa 1 i ni ty of the i rri gated soil sin the Di stri ct generally <br />ranges from a trace to 8.0 mi 11 imhos per centimeter (Mmhos/cm) for <br />most of the soi 1 seri es present, but up to 15.0 Mmhos/cm for some. <br />[5]. It is apparent from field inspection that some lands in the <br />western portion of the District are adversely affected by salinity. <br />Thi s is evi denced by the occurrence of salt crusts on furrows and <br />borders and irregular crop growth within fields. Such visible indi- <br />cations of soil salinity are seldom found in the eastern part of the <br />District, except to a small degree in the southern portion adjacent to <br />the ri ver. <br /> <br />Ground Water <br />The water table lies at an average depth of about 7 feet beneath <br />the flood plain under the District. Ground-water gradients are gener- <br />ally to the south along the direction of the river, except for local- <br />ized variations near the river and major drains. The ground-water <br />reservoir underlying the Palo Verde Valley is hydraulically continuous <br />with the river. [4] Except for local occurrences of clays, water can <br />move freely both vertically and horizontally through the units of the <br />Co 1 orado Ri ver all uvi urn above the Bouse formation. The Bouse forma- <br />tion is composed of older alluvium deposited in a marine to brackish <br />water envi ronment, and is tight enough to cause artes i an pressure <br />beneath it at some places in the region. <br />Information on subsurface water quality has been compiled by the <br />Geological Survey [4], Bookman-Edmonston Engineering, Inc. [2], and <br />the Water and Power Resources Service. The Geological Survey data <br /> <br />(' 'J ~ 8 ,1 G <br /> <br />8 <br />