(Jan. -Dec. total = 603.7 MCM) were nearly 

 as high as the 1980 record, but the 

 distribution differed. The 1980 flood peaked 

 in February, while the 1983 flows were 

 highest in March and remained high through 

 summer and fall. 



While floods are the exception in the 

 streamflow record, they have an enormous 

 influence on the estuary. Not only does the 

 river change its course with some floods, but 

 sedimentation fills channels, raises the level 

 of the mudflats and marsh plain, and gradually 

 reduces the tidal prism (Section 2.4.3). 

 Because the tidal prism is now small, 

 freshwater input during storm events can 

 have far-reaching effects on the system. 

 From the floods of 1978 and 1980, we have 

 learned how dramatically water salinities 

 change, how rapidly the invertebrate 

 populations shift, and how extensively the 

 marsh soils and vegetation are affected by 

 these unusual events (cf. Chapter 5). Though 

 floods are usually brief, they are catastrophic 

 in their effect. What we see at any given time 

 is the estuary's cumulative response since the 

 last major event. 



2.4.2 Wastewater Inflows 



1988, an interceptor was built to collect and 

 return flows down Smuggler's Gulch, but it 

 has overflowed on numerous occasions. 



Recent U.S. projects have reduced the 

 threat of sewage contamination considerably. 

 An interceptor on the Tijuana River was 

 completed in early October 1991. A sewage 

 treatment plant is being planned for the U.S. 

 side of the border, and a new ocean outfall is 

 under evaluation. Final solutions await the 

 impact analysis and permitting procedures. 



2.4.3 Tidal Prism 



The estuary has been characterized as 

 having a low tidal prism, with a range of from 

 0.1 to 3.7 MCM (100-3,000 acre-ft; IBWC 

 1976). However, the terminology is 

 confusing, and it is not always clear what is 

 being estimated. Diurnal tidal prisms are the 

 volume between MLLW and MHHW. Mean tidal 

 prisms are lower than diurnal tidal prisms, 

 and extreme monthly tidal prisms are higher 

 than both of the above. In addition, 

 calculations of tidal prism from topography 

 (potential tidal prism) exceed those 

 determined by measuring flows (effective 

 tidal prism). 



From the early 1980's through 1991, 

 wastewater flows became a continuous 

 problem at Tijuana Estuary. Almost all of the 

 polluted water came from the City of Tijuana, 

 Baja California, Mexico. This upstream 

 metropolis produces far more sewage than its 

 wastewater system can handle. It has a 

 population of over a million, and a doubling 

 time of about ten years. 



In 1988, about 30 MGD of sewage was 

 produced by Tijuana and only 17 MGD 

 collected; the remainder (-13 MGD) flowed 

 down Tijuana River toward the estuary 

 (Seamans 1988). On occasion, there have 

 been breaks in the Tijuana sewer line, which 

 carries the collected sewage to an ocean 

 discharge. Depending on the location of the 

 break, sewage flowed down Smugglers' Gulch 

 and entered the Tijuana River or down Goat 

 Canyon and directly into the nontidal salt 

 marsh at the southern end of the estuary. In 



C 

 CO 



c 



03 



< 



10-1 



3 

 O 



ONDJFMAMJJAS 



Month 



Figure 2.13. Average monthly streamflow at 

 Tijuana River. Asterisks indicate months 

 with average flows <100 acre-ft (Nestor 

 gage; data from USGS 1937-1978). 



22 



