I. Intro. SALINITY ALTERATIONS
We believe that alterations in freshwater flows upstream and impediments to salt water movement at the mouth of the causeway have affected the zone of fresh and salt water mixing.
The relationship of flows to brackishness in the delta can be simply expressed: as the volume of fresh water that flows into the delta increases above about 30,000 cfs, the zone of brackishness moves farther south. But below 30,000 cfs, a number of other factors, including anthropogenic changes, have to be considered.
((Ultimately, three major factors determine water levels in the lower Delta: tides, wind (accompanied by the broader influence of high and low pressure systems), and high water flows. As a result, dramatically low water flows rarely result in a decrease in overall water level in the tidally influenced grassland Delta: When freshwater levels fall, there might be a brief disequilibrium, but the sheer volume of water in the bay, with the backing of all the world's oceans, should quickly bring the overall water level in the Delta back up to its position relative to the tide, and it does that by replacing freshwater content with saltwater content.
Wind and weather influences would generally support and amplify these trends: the wind is frequently from the south when freshwater levels are lowest; the wind is often from the north when freshwater levels are highest.
Fluctations in barometric pressure and prevailing wind patterns in the Gulf can cause a "set-up" of Gulf waters, actually raising the level of the waters of Mobile Bay, and increasing the hydraulic head of tidal inflow. Wiley Kitchens, who has worked with Fish and Wildlife on Delta and estuary restoration in Fla., La., Ga., and elsewhere, points out that wind- and weather-influenced fluctuations may be as important as tidal fluctuations in systems with low tidal ranges.
Other studies have shown that the degree of salt and freshwater mixing in Mobile Bay and the location of isohalines is primarily dependent on inflows, and secondarily on wind and wave action. (See Salinity Characteristics of Gulf of Mexico Estuaries, Orlando, Rozas and Klein. 1993. NOAA publications, pp. 75-79.) As this study shows, in the bay the degree of salinity response is dominated by monthly, seasonal and annual changes in freshwater inflow; daily to weekly alterations in inflow are considered secondary. On a monthly/seasonal timescale, "inflow maintains near-fresh conditions in distributaries, but allows significant Gulf influence under low-inflow conditions."
In the NOAA study, wind and weather patterns are considered an important secondary influence on a daily to weekly time scale and minor on a monthly to seasonal timescale. Tides, by contrast, are considered a minor influence largely resticted to the lower bay. Density currents (associated with channels) may increase bottom salinity reponse in the Mobile channel, but this reminas a relatively isolated response.
Isphording, in his "Sedimentological Study of D'Olive Bay," (Army Corps of Engineers contract; 1981) indicates that the "break-point" of low water flows is about 35,000 cfs — below that point "water height fluctuations in Mobile Bay may substantially override the hydraulic head of any of the distributaries, causing major changes in flow characteristics." About a fourth of the flows into the delta are below this point; as flows continue to decline below 35,000 cfs, the Gulf influence becomes more dominant, and the likelihood of widespread salinities above 1 ppt increases in the delta.))
But the fact that salt water is heavier than fresh water in some measure complicates this relationship: fresh water flows are likely to "skate" over the salt water for great distances before any substantial mixing occurs (and mixing is largely advective, resulting from the friction between the opposing fresh and saline currents). For the same reason, fresh water flows would have to be very small, and of very low velocity, to allow intrusion of salt water or significant mixing at the surface above the causeway (and as we'll attempt to demonstrate, the surficial intrusion of salt water has far broader impacts on the estuary than higher salinity zones in the bottom of the channels).
Still, it's evident such low-water conditions do exist occasionally in summer: The Alabama Department of Environmental Management (ADEM) blames low dissolved oxygen conditions in the Mobile River, in part, on the intrusion of salt water in summer. But for our purposes, the Tensaw channel would be more representative, as salt water intrusion there has no help from channelization, and we will in due order describe a high salinity event in that body of water. (ADEM has offered us data that inadvertently track monthly surface salinity trends at 4 points in the lower delta for a 20-year period beginning in the early 1970s; what we probably won't be able to find is a neatly packaged pre-1930, pre-causeway, pre-dam baseline for salinity trends. We welcome suggestions.)
NOAA's "Salinity Characteristics of Gulf of Mexico Estuaries" offers an isolated look at contemporary trends: This study (citing Bault) characterized salt water intrusion July-Oct. 1968, when in-flows were 15 percent below long-term averages for that season. Salinities peaked in August, but were higher throughout the period.
"Brackish salinities penetrated deep into the distributary system, affecting both surface and bottom layers," the authors write. Surface salinities as high as 5 parts per thousand are registered at Blakeley Bluff, 3 to 4 miles up in the heart of the scrub forest region on the eastern side of the delta. But there is a strong latitidudinal gradient in salinities moving northwest through the upper part of Grand Bay to the Mobile River.
The study clearly shows that salt water intrusion is at least an occasional seasonal event in the Delta, but because it records only a single annual event, it is limited on two counts: 1. It was conducted before the dams doubled their storage capacity upstream. 2. It offers no clue as to whether this salt water intrusion (and the latitudinal gradient) is greater than or less than historical norms (particularly, pre-causeway norms). In addition, the survey points appear to overlook areas outside the remaining open channels, so we have little information on the degree of salt water mixing in the larger bays like Chocolatta (which are now "shut in" by the causeway).
Using the ADEM monthly salinity figures for the period July 1978 to November 1996 (surface salinity was measured one day each month, with data from 8 of those months missing), we counted a total of 22 months of surface salinity events greater than 1 ppt four miles into the delta at Gravine Island on Tensaw River, just north of the area where Bault recorded 5 ppt in 1968.
Only five of those months indicated "events" 3 ppt or greater, and none were above 5 ppt.
Surface salinities at two stations on the Mobile River, on the other hand, were much higher throughout the recording period. At Government Street, nearly every month showed salinities above 1 ppt, salinities above 5 ppt accounted for roughly half the days, and salinities above 10 ppt were frequent. The high readings at these stations suggest the influence of the ship channel, and are not likely representative of readings elsewhere in the delta.
Because these figures represent only one day a month, there's a good likelihood that "salinity events" were actually more frequent and possibly of a greater magnitude at all sites during the 1978-1996 POR, though we expect they would follow similar patterns of occurrence.
In addition, we have prepared a map of salinity readings thoughout the upper bay and delta in 1995, using a new low-flow season ADEM sampling process initiated in 1994 (see "Salinity readings" appended). These readings spanned a variety of sites — both within the channels and in shallow river edges, though none were taken in the interior basins or in the midst of the marshes. During the July 1995 "salinity event," surface salinities ranged from 2.9 to 4.1 ppt across much of the lower delta marshland area. Salinities in the area apparently continued to increase through September. This was the only significant salinity event recorded in the eastern section of the Delta this decade.
But we assert that whatever the contemporary trend is in salt and fresh water interchange, it has been altered at both ends, first by the causeway, and later by increases in freshwater inflow during the low flow season.
© 2012 Alabama Live LLC. All rights reserved (About Us).
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of Alabama Live LLC.
Community Rules apply to all content you upload or otherwise submit to this site. Contact interactivity management.