Evaporation was no doubt high throughout the 

 spring (Section 2.2) and summer, making 

 soils hypersaline by September 1980; thus 

 the period of brackish soils was short-lived. 



• Soil salinity was again low (30 ppt) in 

 April 1983, when rainfall occurred late in 

 spring. The average for April underestimated 

 salinities later in the growing season. Covin 

 (1984) sampled his experimental plots in the 

 lower marsh and documented mean salinities 

 of 31 ppt in April, 29 ppt in May, 39 ppt in 

 June, 33 ppt in August and 34 ppt in 

 September. Major influxes of fresh water 

 from Rodriguez Dam occurred throughout the 

 growing season (IBWC 1983) that dropped 

 channel water salinities to zero in August 

 (Zedler et al. 1984a). 



1978 1980 1982 1984 1986 1988 



Figure 5.2. Changes in interstitial soil 

 salinity in the cordgrass-dominated 

 monitoring stations (n = 102). Data are 

 parts per thousand (ppt) in April, following 

 winter rainfall, and in September, following 

 the long dry season. Samples are from the top 

 10 cm of soil. Vertical bars (mostly too 

 narrow to show) are +1 standard error. 



• An average soil salinity of 104 ppt in 

 September 1984 resulted from estuarine 

 closure in April 1984 followed by 

 evaporation throughout the hot, dry summer. 

 Again, this is an underestimate of salinity, 

 because several readings exceeded the 0-150 

 ppt refractometer scale. Soil moisture at the 

 end of summer was extremely low; channels 

 were dried and cracked. The water table was 

 30 cm below the surface on May 31, 1984, 

 and more than 75 cm below the surface on 

 October 9, 1984. 



5.2.2 Sedimentation in the Salt Marsh 



The monitoring program tracked changes 

 in elevation within Oneonta Slough, and these 

 are reported below. A larger view of the 

 impacts of flooding in 1980 followed an 

 evaluation of aerial photos before (1976) and 

 after (1986) the catastrophic flood 

 (Williams and Swanson 1987). Results of 

 that analysis are presented in Chapter 6, 

 along with plans for removing sediments and 

 restoring tidal flushing to the southern half of 

 the estuary. Because Oneonta Slough is not 

 directly in the path of the river or the steep 

 canyons that abut Mexico, most of the 

 sediments did not reach the cordgrass marsh. 

 In fact, the persistence of the remaining 

 cordgrass marsh is a function of its location 

 and greater protection from sedimentation 

 events. Williams and Swanson (1987) 

 estimated that up to 2 m of sediments were 

 deposited at the foot of Goat Canyon. 



Elevations within the cordgrass marsh 

 increased an average of 5 cm after the 1980 

 flood (Table 5.2). Most of the accretion was 

 due to sediment deposition, but rafts of wood 

 and plastic also accumulated in the lower 

 marsh. Later, during the 1983 winter, the 

 estuary mouth closed briefly and a large 

 volume of sea water was impounded for 

 several days. Sea levels were much higher 

 during the January 1983 storm than during 

 previous tidal maxima (Figure 2.2; Cayan and 

 Flick 1985). Debris that had accumulated in 

 1980 floated to the high ground along the east 

 edge of the salt marsh. 



101 



