The model indicates that high salinity water (30 ppt) should be present 

 throughout the lower estuary from Northport/Harborside southward (figure 5- 

 19), which compares well with Siewell's data. Salinity decreases rapidly from 

 the region of Sandy Point northward with 15 ppt at Bucksport, which agrees 

 well with an observed 14 ppt (Garside, unpublished ) , especially since large 

 tidal excursions are to be expected based on the high tidal currents predicted 

 for the upper narrow section of the estuary. The nontidal currents are weak 

 in the lower part of the estuary and give rise to flushing times that are 

 extremely long when the high fresh water input is considered. 



According to data collected by Haefner (1967), the seasonal temperature cycle 

 of the Penobscot estuary probably is typical of Maine estuaries. Warming of 

 the fresh water inflow and the surface results in a seasonal thermocline 

 (layer of water separating warmer, lighter water and colder, heavier water) 

 forming in April and persisting until October. Temperatures decrease with 

 depth and distance downstream, with the highest temperatures (68 to 72° F; 20 

 to 22° C) in the upper part of the estuary in August. In winter the freeze 

 begins in December and persists until March in the upper estuary. In April 

 uniformly cold (39 to 4l°F; 4 to 5°C) water exists at all depths from Bangor 

 to Bucksport, with the coldest (360F; 20C) water in the bottom downstream from 

 Verona Island. 



The Center for Natural Areas (1978) has compiled a summary of hydrographic 

 data on Penobscot Bay that corresponds to the lower part of the Ketchum model 

 of the estuary. Water column temperature ranges were highest in shallow coves 

 and protected embayments in April (36 to 43°F; 2 to 6°C) and in August (55 to 

 72° F; 13 to 22°C) with a lower range in deeper open waters. During winter 

 months, temperatures were uniform at 360F (2°C) in the upper 39 feet (12 m) . 



In summer the temperature at Im reached a seasonal maximum of 64^ (18 C) in 

 July and at 10m, 54 F (12°C) in September. A distinct thermocline usually 

 occurs from May to late August. The thermocline forms when surface 

 temperatures are about 46°F (8°C) and disapoears in the fall when surface 

 temperatures are about 57 to 61 F (14 to 16 C). 



During periods of high runoff, the upper 10m of the water column has 

 diminished salinity (as low as 20 ppt) underlain by 30 ppt water. Under low 

 flow, vertical salinities are much more uniform, with 28 to 30 ppt throughout 

 the water column. 



Density stratification varies in response to both temperature and fresh water 

 flow, so that warming temperatures and high flow in the spring give highest 

 stratification while low temperatures and low flow in winter result in least 

 stratification. Haefner (1967) considers the density structure in the 

 Penobscot to be typical of a partially mixed or moderately stratified estuary. 



Currents in both the upper estuary and Penobscot Bay (lower estuary) are 

 dominated by tidal flows. The high tidal flow in the upper estuary (figure 

 5-19) under low fresh water flow conditions could provide the energy for the 

 complete vertical mixing noted by Haefner (1967). Tidal flows in the lower 

 estuary are much less, and in the deeper water will contribute less mixing 

 energy and allow stratification to be maintained. Nontidal flow affects 

 flushing in the upper estuary, where the first 12 miles (20 km) flushes in 2 



5-32 



