City of Franklin Considers More Drinking Water Withdrawals from Harpeth

How Much Water Does a River Need?

By Pam Davee, Director of Development and Policy Specialist

When you look at a river, how do you know it has enough water in it to do what it is supposed to do during that time of year? What is it even supposed to do at various times of year? Well, that depends. Why are we even asking? Because these very questions need to be answered right now for the Harpeth as the City of Franklin contemplates more water withdrawals from the river to meet its growing drinking water needs.

Spring and Fall are Critical Times Eastern Tennessee rivers like the Harpeth, which get very low in the summer, naturally have less water in them during these months than during others. It is part of the river’s natural flow cycle. Low flow, as much as high flow, provides cues for certain lifecycle events such as spawning and egg hatching.

During the high flow months the river stores water in its banks and recharges groundwater as well as allowing certain fish and other aquatic life like mussels and snails sufficient depth, habitat, and flow to reproduce.

In the low flow months the river is more vulnerable to degradation because the water depth is shallow. Thus, the river is susceptible to higher temperatures especially if it does not have an adequate riparian buffer (i.e. streambank trees and other vegetation) shading it from the sun.

If the water temperature gets too high and too much sunlight shines on the shallow water, it can cause too much algae to grow, which can lead to low oxygen levels (a process called eutrophication) and potentially lead to fish kills. The warm water also encourages bacterial growth, which is a particular problem for urban rivers which receive significant amounts of stormwater runoff that is laden with bacteria. The bacteria have a perfect environment in which to multiply if the river is experiencing its annual low flow.

Therefore, the summer and early fall are critical times to keep enough water in rivers like the Harpeth, especially when demands increase for water for golf courses, lawns, pools and agriculture not to mention for drinking. This is the time of year when the chemistry of the river can be dramatically altered by low flow, higher temperatures, increased runoff and sewage treatment effluent (waste water).

Keeping adequate flow in a river and calculating how much water can be removed at any given time without adversely altering the river system and its natural functions and processes are two important inter-related issues that scientists have begun to study in earnest in the past few years. The area of study is called Instream Flow.

Instream Flow & River Health

According to the Instream Flow Council (IFC), the concept of instream flow is simple. It means the water flowing in a stream. Most streams have some level of flow, but flow is no guarantee that all is well for the organisms (including humans) that depend on the river’s resources. At the dawn of civilization, instream flows were adequate to sustain the full range of river dependent organisms and processes. But as civilization advanced, competing uses by humans have altered, diminished and contaminated original riverine uses, according to the IFC’s 2002 book, Instream Flows for Riverine Resource Stewardship .

River health can only be maintained by preserving the processes and functions of the river ecosystem. The IFC further explains that the structure and function of river systems are based on five river components:

_ biology,

_ water quality,

_ hydrology (river flow),

_ connectivity (the maintenance of pathways for different biological and other processes) and

_ geomorphology (the shape of the surface of the earth). An evaluation of a river’s flow to determine if an alteration of it will negatively affect the system generally examines all of these components.

The City of Franklin is currently considering additional withdrawals from the Harpeth for the drinking water plant to partially meet the City’s growth projections. Consequently, the City hired a consulting firm, Consoer Townsend Envirodyne (CTE), that HRWA is working with closely to examine critical low flow functions.

It cannot be assumed that further reduction in water levels during the low flow season will not interfere with one of these five river functions. Once it is established whether or not the Harpeth can accommodate further withdrawals and at what time of year, the City will be able to move forward with its planning process to provide for the future water needs of Franklin residents without degrading the Harpeth.

If the river cannot provide the needed additional water in an environmentally and economically sound manner, then other alternatives will be explored, such as additional purchases from the Harpeth Valley Utility District that provides drinking water from the Cumberland River and already supplies the majority of the City’s water.

In the Harpeth during various times of the year the river level pulses so that during a single year and over the course of several years the river swells and contracts. This has an impact on organisms in the river as well as those in the riparian zone that surrounds it. These riparian areas are connected to other ecosystems and are maintained by groundwater and flood pulses.

Part of a flow study would include examining the river’s historic flow levels (hydrology) by examining US Geological Survey data. The importance of the natural hydrograph (graph showing the variation in flow over time) to a stream has been demonstrated by the outcomes from water studies where activities altered the variability of the flow and damaged the ecological system. This tells us that streams need to ebb and flow throughout the year to maintain a healthy overall river system and not have their variability removed by constant, nonvariable withdrawals.

Additionally, the current City of Franklin study is examining the geomorphology of the river by looking in the channels and banks of the river to see how flow is shaping them with the creation of riffles, pools and other features.

One of the other aspects the scientists and engineers are examining is what benthic macro-invertebrates (water critters) and fish live in the channels, riffles, and pools of the river. This is the beginning of a biological assessment that examines: What is the composition of the biological communities? What species, aquatic and terrestrial, are likely to be affected by changes in natural flow? Should particular species be targeted for protection — like game fish or rare species? And finally, to complete the assessment of the Harpeth’s five riverine components, the study will look at water quality with measurement of water chemistry and physical and biological properties of the water.

Dissolved Oxygen (DO) and levels of alkalinity, nitrogen and pH reflect local land use, geography and climate and sources of organic matter and determine the river’s biological productivity. Temperature is also an important component because it affects all forms of aquatic life.

What We Need to Know

Because the Harpeth is already not meeting its designated uses in the mainstem (the main part of the river that runs through downtown Franklin where the additional withdrawals will have the most impact), the City will have to demonstrate that any water withdrawal program does not degrade this section of the river or it will not be permitted by the Tennessee Department of Environment and Conservation (TDEC).

HRWA is not only working with the city, but with TDEC, other agencies, and organizations on the state’s instream flow and water withdrawal policies as more communities around the state wrestle with how to provide for expanding water needs. Much data about the Harpeth River has already been collected by TDEC, the Tennessee Wildlife Resource Agency (TWRA), the United States Geological Survey (USGS) and the Environmental Protection Agency (EPA), all of which are working closely with the City as the result of our efforts in the assessment process. Existing data has been examined and CTE is conducting the necessary field assessments to fill in the gaps.

So, to answer the question of how much water does a river need — the answer is: just enough to do everything it needs to do to function as a healthy river system based on all of the ecological interactions required of it at that time of year. It is seasonal. It is variable, and it is complicated, but the answers are knowable based on what has been learned in recent years. With the work that the City is currently doing to assess the Harpeth we can all benefit from the latest in the study of instream flow.

-From Voices for the Harpeth, Fall 2005