Minimum Flows and Levels

What the Law Says About Minimum Flows and Levels (MFLs)

Florida law requires the water management districts to establish MFLs for surface waters and aquifers within their jurisdiction (section 373.042(1), F.S.) The minimum flow is defined as the "...limit at which further withdrawals would be significantly harmful to the water resources or ecology of the area". The minimum level is defined as the "limit at which further withdrawals would be significantly harmful to the water resources of the area" (section 373.042(1), F.S.). The statute further directs water management districts to use the best available information in establishing the MFL level. Each water management district must also consider, and at its discretion may provide for, the protection of non-consumptive uses in the establishment of MFLs (section 373.042, F.S.) In addition, a baseline condition for the protected resource functions must be identified through consideration of changes and structural alterations in the hydrologic system (section 373.042(1), F.S.)

These MFLs are being developed pursuant to the requirements contained within the "Florida Water Resources Act", and specifically, Sections 373.042 and 373.0421, F.S., as part of a comprehensive water resources management approach geared towards assuring the sustainability of the water resources.

The proposed MFL's are not a "stand alone" resource protection tool, but should be considered in conjunction with all other resource protection responsibilities granted to the water management districts by law. This includes consumptive use permitting, water shortage management, and water reservations. A model framework identifying the relationship between these tools is discussed in this document and was used in developing the MFLs. In addition, the District has completed Regional Water Supply Plans pursuant to Chapter 373.0361 F.S., which also include recommendations for establishment of minimum flows and recovery and prevention strategies.

In Practicable Terms ...

However, establishing minimum flows and levels alone will not be sufficient to maintain a sustainable resource or protect it from significant harm during the broad range of water conditions occurring in the managed system. Setting a minimum flow is to be viewed as a starting point to define water needs for sustainability. The necessary hydrologic regime for restoration of defined priority water bodies includes the use of water reservations and other water resource protection tools. Achieving the required water levels throughout these systems is an overall, long-term restoration goal.

Before MFLs are calculated, scientific determination of how changes to flows and levels influence the preservation of water quality in a water body should be made to guard against having catastrophic effects upon a local economy.

Difficult issues attend use of MFLs as a tool for water management.

Local Uses First. An additional Rule of Law is "local uses first". Meaning, that a district must first apply its naturally occurring water resource before it calls upon resources from another district, or another district may apply to use that naturally occurring water resource.

However, a danger is that MFLs for a given resource in times of plenty may lead to a proposition to extract water "surplus" to the MFL, and move it into a man made facility, such as a reservoir, for use to replenish supplies in times of shortage. This may appear to be a good thing until another district applies to use the water from the reservoir as it is no longer considered a "natural" resource.

Furthermore, taking water from underground into a reservoir usually diminishes the resource by allowing volumes to evaporate back into the atmosphere.

Stream Flow and Algae. Algae formations occur more readily in slower moving and stagnant waters than in faster flowing streams.

To determine the stream velocity at which algae of any chosen pernicious species would likely be inhibited, according to presence of given limiting nutrients, itedss not easy. Nevertheless, algae formations degrade water quality and their inhibition should be reflected in MFL calculations.

Combination of Flows into a Water Body. When a water body, such as a river system, has many feed sources, any one or more of the feeds can have adverse impact upon that water body.

Consider Crystal River/Kings Bay, for example. Thirty named springs and very many more unnamed vents supply water to that system. A balance of clean fresh and salt water is required in suitable quantities in estuarine areas for promotion of healthy sea grass meadows, which provide summer habitat for the protected manatee population, and, in addition, support the nurture and diversity of both recreational and commercial fish stocks. The sea grasses have become established in biomass there over many decades according to the water mix. Adverse effects promoted by changes to the balance of that mix degrade quality of the sea grass meadows. Such degradation remains in effect for long periods of time. The dependent fishing and eco-tourism industries are vital to the region's economic health. "Loose the grasses and the manatees go".

Moreover, the health of those meadows depend upon levels of nutrients conveyed to the river in watershed and springshed (surface and underground) flows. The levels which actually reach the estuary are governed by the mix of submersed aquatic and exotic plants which consume them on their way to the estuary. These issues also should be reflected on the calculation of MFLs.

Salt/Fresh water interface. Fresh water is lighter than salt water. Therefore, fresh water "floats" on top of salt water. The weight of the rain water that percolates into the ground depresses the salt water beneath it forming a profile that has the appearance of a lens, which is called the Ghyben-Herzberg lens after those who discovered the phenomenon. It is generally only in shoreline regions that salt water intrudes into the cracks, crevices, and loose rock spaces. It has no meaning where an inland body of ground water may be confined by layers and dikes of karst rock.

The relative densities of fresh and salt water is in the ratio of 40:41. The principle of this relationship essentially states that for every foot of ground water above sea level there are forty feet of fresh water underground. Understand that this applies only to fresh ground water that is sitting directly on an intruded body of salt water.

The underground boundary that separates the fresh water layer from the salt water is not a sharp boundary line, but a transition zone of brackish water (fresh/salt mixture). This is caused by seasonal fluctuations in rainfall, tidal action, and the amount of water being withdrawn either by humans or by natural discharge.

When the weight of over-bearing fresh water is reduced by drought or human up-take, reducing the aquifer head pressure, an up welling of the saltier water from underneath can occur. Similarly, where the aquifer extends under the sea, the water in the aquifer there is generally salt. Diurnal tides cause the salt water to flow into a river system and move a wedge of fresher water inland. As the wedge move inlande the depth of fresher waer is reduced further allowing upwelling of salty water to occur. There is evidence of this happening in Kings Bay today, changing local ecosystems dramatically. (A report of the reduction of the level in Lake Rousseau in the seventies to allow mechanical harvesting of Hydrilla from around the shoreline promoted a nearby major spring vent to issue salt water instead if its traditional fresh water).

Since the resulting variation in saltiness has such profound effects upon local ecology provision should be made for measurements to be allowed for in calculating MFLs.

Effects of distant water sources. Waters emerging from the aquifer as spring discharges often travel long distances over extended periods of time, through underground conduits before emerging at the surface. Quality and stream velocity of the emerging waters may well be influenced by events several miles away.

An example of this most probably affects the principal source of fresh water entering the Crystal River/Kings Bay today. Moreover, discharges from several spring outlets may have their feed from the same remote source. The collection of springs comprising Hunter, Catfish, Magnolia, Idiots Delight and Three Sisters Springs, which have maintained fresh water characteristics while others have not, is a case in point. These fresh water feeds are thought to come from supplies from Levy County by way of the Rainbow River and Lake Rousseau.

Such phenomena of high potential impact need to be reflected in MFL calculations.

Click here to also see news item by Sam Lyons.