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56 F, ZAMORA-ARROYO ET AL REGENERATION OF TREES IN RESPONSE TO FLOOD RELEASES Soil samples and ground-water monitoring o Morel.. Dam FIoros ValeDa..l. Cinco de Mayo Okcnon SuLuis D =i~~ ,. ~onh Rail Road Bridg. I Cann~ Jesus Gonzales Qrtep Ena CouonwoodlWillows o SIlO 1000 ISOO M..... Three soil samples from the top 20 cm of soil profile in each zone (n = 42) were analysed for % sand, silt and clay and electrical conductivity (BC) of a 1: 1 extract by Laboratory Consultants, Inc., Tempe, Arizona. Soil texture class was determined by the proportion of sand, silt and clay in each sample. Well points (5 cm diameter steel tubes with a perforated sand point at the tip) were installed into the water table at or near four of the transects (2-3 per transect spaced ca. 100 m apart perpendicular to the river, 10 total, plus two additional points at Campo Munoz, in the tidally influenced portion of river) to monitor ground-water. Water depth was determined after pumping three or more volumes of water from the casing with a hand pump then allowing the well to recharge; a sample was measured for electrical conductivity (BC) by the U.S. Geological Survey lab, Tucson, Arizona. Well points were sampled in November 1999 and January and February 2000. & jso ~ 100 SIlO 10 Flores Valenzuela Soil a 28'6% 25'0% j5 3422 :i SAND. &53 (0-58) cIS M-I 62 m 0'10 44 10 Results Transect results CLAY LOAM. 1'83 (()o7l)dS M' A summary of soil, ground-water and vegetation conditions in each stratum of each transect is in Fig. 4. The predominant soil type over all transects was sandy loam. BC of most soil samples was low (mean = 0'65 dSm-I). From November 1999 to February 2000 groundwater was shallow (1-2 m) at all stations, but a salinity gradient was apparent from south to north within the native tree zone, with ground-water BC's ranging from 1.4 to 4'4 dS m - I for well points within the native tree zone. (Two well points placed below the junction with the Hardy River at Campo Munoz had water of 9'7 dS m - I). Ground-water in general was saltier (higher BC) than river water (1' 2 dS m - ]), presumably due to evaporation, the influence of saline subsurface drain- age from adjacent agricultural fields and to the flushing of salts from the soil surface to the groundwater by flood waters. Five plant species were commonly encountered in study plots (Fig. 4): T. ramosissima, a mid-story species (up to 6 m height); Pluchea sericea (arrowweed), a salHolerant shrub; Baccharis salicifolia (seepwillow), a mesophytic shrub; S. gooddingii and P. fremontii trees, that were present in all size classes but were the only species above 6 m height; and the emergent, aquatic grass, Phragmites australis (common reed), found along the water line on three transects where the channel had not entrenched. Prosopis pubescens (screwbean mesquite tree) was locally abundant in some parts of the floodplain but was not encountered in the transects. Felger et al. (1998) provide a complete flora of the delta. 10 j 5 o SILTY LOAM. &33 (()o12) cIS M- 10 ~ Figure 4. Vegetation distribution along nine transects in the Colorado River delta, Mexico. The location of the transects along the river (y-axis) and length of each transect (x-axis) are shown schematically in the upper left hand graph. Some transects were divided into separate zones, results of which are shown separately in the pictographs that follow. The locator graph also shows whether transects were on the east or west side of the river channel. Symbols for individual plant types are shown in the upper right hand box using common names. In the graphs showing results for each zone, the height of the plant symbols indicates plant height (y-axis), while the width of the plant stand indicates % cover. Over each set of plant symbols, % cover (numerator) and density in plants per ha (denominator) is given. Near the name of each transect, the % covered by bare soil is given. The x-axis also indicates the length of the zone in meters, and under the x-axis, the soil type and, to the right, mean and standard error of soil EC in I : 1 extracts is given. In those transect with well points, the mean and standard error of EC readings in the water table is given under the x-axis near the origin to the left of soil type. Note that well points indicated as in the Caranza transect were actually several kilometers distant (see Fig. 1). 15 1'4~(&19)cIS M.I SILTYiA'M.0-23(0-12)clSM-~ m 2'67~I'15)cIS M-' LOo\M.0-.43 (O-40)dS M" 83.. J! J'ji ll,tl'o# ", i ~l c;. CJ W l''':' 10 CD SANDY LOAM. &30 (&30) cIS M-' 92 m Nol1b of Railrvad Zone J Soil- 35',3% 15 9"8% 3i43 54-9% i4.i2i SANDY LOAM. &37 (0-29) cIS M-I 10 North of Railroad 2Dnc II Soil- 35-3% 76'4"'. lli8 SANDY LOAM. 1'45 (&07) cIS M-' 102 m 462m CLAY. 5~ (S~) cIS M- 12'3m 0'10 m 5S4 m 2-58(~.80) cIS M"SILTY LOAM. 0-.10 (D-30) dS M-'S6 m 10 i 5 :IE o 3703'10 3O:2i6 LOo\My SAND. &23 cIS MO' 38 m 10 LOAMY SAND. D-24 (0'06) lIS M-' 1422 m