currently hypothesize that low winter 

 temperatures act to slow terrestrial 

 losses of detritus and nutrients 

 until the spring thaw at which time 

 these materials are deposited in the 

 river in larger than normal quanti- 

 ties. Heinle et al. (1976) suggested 

 that ice-scour of marshes in the 

 Patuxent River was more complete in 

 cold winters and served as an addi- 

 tional organic matter source for zoo- 

 plankton which in turn were more 

 available to first-feeding larvae. 

 River flow may act through several 

 mechanisms and, at this juncture, we 

 are uncertain as to the relative 

 importance of these. One concept has 

 it that higher than normal spring 

 flows transport detritus and nu- 

 trients to the spawning area in 

 greater abundance than in lower flow- 

 years. The enhanced load supports 

 densities of zooplankton at levels 

 appropriate for first-feeding larvae. 

 Another concept extends the first, 

 and suggests that higher than normal 

 spring flows support higher zoo- 

 plankton densities but also expands 

 the area of the river which has this 

 characteristic. Thus, there is a 

 larger nursery area in which zoo- 

 plankton stocks are above critical 

 densities for first-feeding larvae 

 (Polgar et al. 1978). An emerging 

 view is that there is always some 

 zone of the upper nursery area which 

 has sufficiently high zooplankton 

 stocks to support some recruitment, 

 even in years of average or low flow. 

 In this view, the key to successful 

 recruitment involves the distribution 

 i't spawning adults. Preliminary 

 analyses suggest that certain water 

 temperature patterns in tht spring 

 (which are influenced by river flow) 

 act to delay spawning until adult 

 tish have migrated far up into the 

 spawning area. When spawning does 

 occur, ^merging larvae have suffi- 

 cient time to grow through the 

 critical feeding stages prior to 

 being transported out of the rich 



nursery area. During years of high 

 flow, the area! dimensions of this 

 zone expand. ["hus , while we can 

 suggest and, by inference, support 

 several mechanisms, further refine- 

 ments are obviously needed. 



HYDRAULIC ALTERATIONS 

 OF THE POTOMAC ESTUARY 



The United States Army Corps of 

 Engineers, in response to legislation 

 requiring the development of water 

 supply plans for major Northeast 

 metropolitan regions, has assisted in 

 developing such a plan ior the 

 Washington, D.C. ,ict\i. The overall 

 program, commonly referred to as NEWS 

 2020, is the Northeast Water Supply 

 plan to the year 2020. 



[Tie Washington metropolitan area 



obtains the major portion of its 

 water supply from the Potomac River 

 upstream of Great Falls. As given in 

 Figure 5, the minimum low flow re- 

 corded was 388 million gallons per 

 day (mgd) on September 196b, while 

 the peak summer withdrawal was 488 

 mgd on 18 July 1974. A key issue 

 then is the adequacy of river flow to 

 meet demand during 1 ov How periods. 

 Early alternatives suggested were 

 installation of dams on the mainstem 

 river or tributaries in order to 

 withhold spring excess flow and to 

 release tins water during summer Low 

 flow periods. Subsequent considera- 

 tions were various water conservation 

 scenarios (Figure 5 and Table 3; 

 Water Forum Notes 1978). Although 

 conservation efforts may reduce pro- 

 jected increase demands, there still 

 remains the possibility oi storing a 

 portion of excess spring flows to 

 in.ot. future water needs. If spring 

 I 1 ows are critical to sir iped bass 

 success in the Potomac, it may be 

 that future conflicts will develop 



160 



