high tide occurs later upstream than at the mouth. In an estuary with 

 parallel sides and uniform depth, the tide distributes energy throughout and 

 the tidal amplitude decreases towards the riverine end. In estuaries that 

 become narrower towards the riverine end, the tidal wave is narrowed and 

 becomes higher as it moves upriver. If the tidal wave is high enough, or if 

 the bottom is shallow enough, the tide begins to interact with the bottom and 

 friction slows the wave. The same friction causes turbulence in the water 

 column and results in mixing. If the tide is high enough and the bottom 

 shallow enough, the tidal wave breaks and moves up the estuary as a tidal 

 bore. 



When bottom friction on the tidal exchange is low, turbulence is only 

 sufficient to push salt water up into the layer of fresh water. This process, 

 called entrainment, results in the surface layer becoming increasingly salty 

 as it flows to the sea. (The term brackish water is used to describe this 

 water, which has a lower salinity than coastal sea water.) The underlying sea 

 water is not diluted with fresh water and is saline but moves up the estuary 

 to replace the salt water lost by entrainment. With increased tidal friction, 

 entrainment becomes a two-way process: fresh water mixes downward into the 

 bottom layer and salt water mixes upward into the surface layer. In estuaries 

 in which two-way entrainment takes place, called "partially mixed" or 

 "partially stratified" estuaries, both the surface and bottom layers have 

 decreasing salinities toward the riverine end, and the flow is seaward at the 

 surface and landward at the bottom (figure 5-9). 



When bottom friction becomes extreme, mixing from surface to bottom occurs and 

 the water column becomes homogeneous from top to bottom. Estuaries of this 

 type are termed "vertically homogeneous" or "fully mixed" estuaries, and 

 salinity decreases from the oceanic to the riverine end of the estuary but is 

 uniform throughout the water column. Water moves towards the oceans at all 

 depths and the salinity of the estuaries is maintained by horizontal mixing or 

 diffusive processes (figure 5-9). 



Topography . Although tides provide the energy for mixing, topography 

 determines where that energy is translated into mixing. An estuary with a 

 deep wide mouth will not restrict the entering tide and may approximate the 

 "salt wedge" estuary at that point. As the estuary narrows and becomes 

 shallower, the topography begins to increase the tidal height and mixing, and 

 increasing bottom friction will result in the estuary becoming partially 

 stratified. Still further toward the riverine end, the shallower water and 

 greater bottom friction creat a well-mixed and homogeneous estuary. Thus, 

 all estuarine types and gradations between them can occur in a single estuary, 

 depending on topography. Likewise, spring and neap tides and changes in 

 seasonal freshwater flow can cause a given portion of an estuary to change 

 from one type to another. 



Winds. Prevailing winds affecting large estuaries can modify circulation 

 and tides. Tidal ranges can be altered by winds blowing along the axis of the 

 estuary, and crosswinds in sufficiently large estuaries can result in the 

 seaward flow being moved to one side of the estuary and accelerated greatly or 

 diminished, depending upon the direction of the wind. Storm winds creating 

 surface waves can cause intense vertical mixing. These effects last only as 

 long as the winds, and relaxation is rapid afterwards. However, such wind 

 events are frequent enough that they introduce many short-term variations into 



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