We have no data on nutrient inputs during 

 natural flood events at Tijuana Estuary, and 

 can only speculate that nutrient influxes and 

 accumulation rates are greater during flood 

 than nonflood years. All studies of filtering 

 functions during flooding came after 

 hydrological modifications had changed both 

 the availability of sediments for transport and 

 the rates and timing of streamflows. Onuf's 

 comparisons of flood effects at Mugu Lagoon 

 before and after 1970 indicate that 

 sedimentation was much less during floods 

 that preceded watershed development. The 

 combined floods of 1978 and 1980 filled in 

 the central bay of Mugu Lagoon and decreased 

 low-tide volume by 40% (Onuf 1987). 



Disturbances within the watershed of 

 Tijuana Estuary have destabilized slopes and 

 made available large volumes of sediment, 

 just as in the watershed of Mugu Lagoon. 

 Unlike Mugu Lagoon, Tijuana Estuary is 

 somewhat protected from sediment deposition 

 by dams that regulate 78% of the watershed. 

 Still, agricultural and urban developments 

 below Rodriguez Dam disturb soils that can be 

 mobilized by flooding. Aerial photos of the 

 1980 flood show major sediment plumes 

 flowing out of the mouth of Tijuana Estuary. 

 Only a small portion of the sediment was 

 deposited within the wetland. 



Because the bulk of the filtering process 

 occurs as sedimentation during catastrophic 

 events, it is not clear how important the 

 vascular plants are in controlling the 

 processes of accretion and erosion. In coastal 

 systems worldwide, there are attempts to 

 stabilize shorelines by maintaining good cover 

 of beach grasses and cordgrass. At Tijuana 

 Estuary, the importance of beach vegetation in 

 reducing sediment mobilization is clear, but 

 the role of cordgrass is not. Cordgrass does 

 not occur in the path of the river; thus, it 

 cannot reduce erosion along the river banks. 

 Some areas of pickleweed in the estuary and 



even some of the woody plants upstream were 

 scoured out by the 1980 flood. Cordgrass may 

 be effective in increasing sedimentation 

 within the salt marsh. Whether this is 

 beneficial or detrimental to maintenance of 

 wetland habitats depends on the combined 

 rates of accretion and sea level rise. 



10 

 8 



6 



O 



CD 



E 4. 



2 







a Succulent 



MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR 



1977 1 1978 — ' 



b m 



MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR 



"1977- 



•I — 1978 — ' 



Figure 4.9. Mean concentrations of dissolved 

 organic carbon for flood and ebb tidal waters 

 of a) the succulent-dominated and b) the 

 cordgrass-succulent study sites. Reprinted 

 from Winfield (1980) with permission. 



96 



