225 



Geographic bias can be further reduced by grouping stations into a limited 

 niomber of regions, weighting each region by the area of ocean covered, the 

 shore length, or the relative data reliability. Existing tide stations 

 should be maintained in order to extend the record length. New sites 

 should be located in stable areas that are presently underrepresented. 

 Methods of improving the tide-gage network and linking to satellite 

 altimetry will be described in the discussion of Sect. 3. 



Vertical land motions (glacioisotasy and hydroisostasy, neotectonism, 

 sediment loading) 



Glacioisostatic movements have been removed from U.S. tide records by 

 applying viscoelastic earth models (Peltier, 1986). An alternative 

 approach is to use late Holocene (<6000 yr) sea level curves derived from 

 ■'-^C-dated sea level indicators (Pardi and Newman, 1987). These curves 

 include glacioisostatic and local neotectonic movements. Therefore, 

 subtraction of late Holocene sea level trends from tide -gage trends should 

 minimize these effects (Gornitz et al., 1982; Gornitz and Lebedeff, 1987). 

 The two approaches are not completely independent, in that Peltier uses C 

 data to calibrate his models. 



Sediment loading and lithospheric cooling (Pirazzoli, 1986) are 

 insignificant causes of global sea level change on a 100 -year time scale 

 (Table A. 4). The subsidence rate along the U.S. East Coast continental 

 shelf and margin over the last 12 million years is only around 0.03 mm/yr 

 (Heller et al . , 1982; Steckler and Watts, 1978). The global sea level rise 

 corresponding to present deposition of river sediment on the continental 

 shelves and slopes is only 0.01-0.02 mm/yr (after Milliman and Meade, 1983; 

 Holeman, 1968). These rates are only a fraction of the total observed sea 

 level rise. However, locally, near major deltas (such as the Mississippi 

 delta), relative sea level curves may show much higher rates (e.g. ~9 mm/yr 

 along the Louisiana coast) . 



Anthropogenic activity (dam building, enhanced sedimentation, subsurface 

 fluid withdrawal) 



Impoundment of water behind dams and reservoirs and infiltration of 

 water into aquifers, between 1932 and 1982, are equivalent to withholding a 



