WATER QUALITY AND ITS DEPENDENCY ON THE PRESENCE OF VEGETATION WITHIN THE STREAMBED AND ALONG THE BANKS OF TRIBUTARIES IN THE DOG RIVER WATERSHED

Jessica C. Simmons, Department of Earth Sciences, University of South Alabama, Mobile, AL 36688. Email: kurblot@hotmail.com.

Extensive channelization of the Dog River Watershed has resulted in the separation of riparian vegetation from its tributaries. This has compromised the water quality and overall health of the watershed. Riparian zones and floodplains are important features of the watershed’s ecology and must be reestablished. Vegetation helps to maintain and improve water quality by functioning as a buffer, filtering out sediments and debris. It provides habitats for organisms that contribute to the water’s health, and it creates an obstacle that slows down stream flow, especially after a rain event. This study shows that where vegetation occurs in three of the watershed’s channelized tributaries, Moore Creek, Milkhouse Creek and Montlimar Canal, water quality is enhanced. Water quality tests were conducted at four sites along each tributary; two sites where vegetation came into contact with the water and two sites where no vegetation was present in the water. These tests indicated that water quality is superior where vegetation occurs in the streambed. By allowing plant life to reestablish itself within the channelized stream, water quality can be significantly improved for not only that tributary, but for the entire watershed.

Keywords: water quality, channelization, riparian vegetation.

Introduction:

Due to the extensive channelization of the Dog River Watershed’s tributaries, riparian vegetation is no longer a prominent part of the watershed’s ecology. This unfortunate fact can affect the health and quality of not only those tributaries in particular, but the entire watershed as a whole. The riparian zones of any stream or tributary function as sponges, absorbing and filtering out sediments and other debris, as well as “providing unique habitats for some organisms” which also contribute to the water’s health (Moore 2003). Because small tributaries have a close relationship between their “terrestrial and aquatic systems”, the separation of these two systems can greatly affect the amounts of “sediments, water, and nutrients” that end up downstream, therefore affecting the overall condition of the watershed (Moore 2003).

The upper reaches of the Dog River Watershed receive all storm-water runoff from the surrounding developed areas, especially from those areas of West Mobile. Due to the rapid development of this area, intense runoff from the numerous impermeable surfaces carries with it large sediment loads that drain into the Mobile basin, “which ends at Dog River” (West Mobile’s muddy waters 1998). Tons of sediments are washing down “from the heights of Mobile’s watershed to the bottom of Mobile Bay” every time it rains (West Mobile’s muddy waters 1998). Although the increased runoff can be attributed to development, it is intensified by the fact that most of the tributaries receiving the runoff have been channelized. The concrete or gabioned streams cannot slow the water or absorb the sediments sufficiently because they can no longer spread onto their floodplains where riparian vegetation would have filtered the load. In turn, water quality is compromised due to the lack of riparian buffers.

Channelization has been utilized as a way to control flooding or divert stream flow for hundreds of years, but its impacts are poorly understood and greatly understudied (Hupp 1992). What is understood, however, is that channelization is clearly a traumatic and stressful experience for a stream. Losing access to the flood plain and receiving a concrete-lined streambed will alter a stream’s ecology in many ways, which will eventually affect the whole watershed system. For example, creating a boundary between the stream and its flood plain can drastically increase stream gradient, which makes it difficult for vegetation to rejuvenate itself and intensifies flash flooding events (Hupp 1992). Also, when vegetation is not able to reach the stream, potential habitats are lost. A study conducted by the Nature Conservancy concerning Alabama’s rivers found that aquatic species are declining rapidly all over the state (“Species are in danger? Reports puts Alabama at the top of the nation in number of aquatic animals imperiled by pollution” 1996). Without vegetation to build a home in, some aquatic organisms find it difficult to live in a channelized stream and therefore cannot contribute to improving water quality. It is evident that without the presence of riparian vegetation, water quality suffers.

One study, for example, observing the vegetation-water quality relationship explained that channelized streams experience “channel evolution” occurring in a series of stages. This study shows that channelized tributaries have the potential to heal themselves without the aid of humans. However, each stage of evolution can take years to transpire and the river will not reach its full recovery and stability for at least another 50 years (Hupp 1992). Until this stabilization is achieved, water quality will be under constant threat.

Riparian vegetation and water quality are clearly dependent upon one another. Channelized streams in the Dog River Watershed, especially those in the upper reaches, must redevelop a relationship with their terrestrial environments in order to maintain water quality for the whole watershed.

Unfortunately, it is difficult to give back the river’s floodplains after channelization and subsequent development have occurred. But by assuring that vegetation is present in the stream beds and along the banks of the Dog River Watershed’s tributaries sediment loads will be more easily absorbed and filtered, habitats will become available, flash flooding can be minimized and downstream systems will not be as severely affected. By reintroducing terrestrial environments to channelized streams early, water quality and overall stream health may be improved for the entire watershed.

Research Questions:

Does the presence of vegetation in the streambed and along the banks of channelized tributaries affect the water quality positively? Is the water quality superior in areas where vegetation occurs as opposed to areas where vegetation does not occur?

Methods:

To establish a water quality profile that would be representative of most of the channelized tributaries within the Dog River Watershed, three creeks were chosen based on their varying stages of channel recovery, to what extent the tributary has been channelized and its location in the watershed. Moore Creek, Milkhouse Creek and Montlimar Canal were investigated as indicated by the red dots in (Figure 1).

First, Moore Creek was chosen because of its location in the middle of the watershed. This tributary receives runoff from both east and west Mobile. Next, Milkhouse Creek was selected because it represents the tributaries that have experienced only partial channelization, where large portions of the creek remain untouched and natural. Finally, Montlimar Canal is representative of those tributaries that have had the bulk of their reaches channelized. It is also in a much later stage of channel recovery being that the canal was constructed in the mid-1950’s following the drainage of Wragg Swamp (Vernon 2002).

Water quality for each creek was obtained and measured using an Alabama Water Watch testing kit. Turbidity, dissolved oxygen levels and air and water temperatures were determined for each site over the course of two weeks during the recommended timeframe for testing; between 10:00 AM and 2:00 PM.

At four separate sites along each of the aforementioned tributaries, water quality was analyzed. Each site within the tributary was selected based on the presence or absence of vegetation in and along the streambed. In the upper reaches of each tributary, two water quality tests were performed, one where vegetation came into contact with the water, and the other where vegetation was not present in the water. In the lower reaches of each tributary, two more tests were carried out in the same method. Testing the full extent of each tributary in this manner ensures a more representative water quality profile for that stream. In summation, each tributary was examined at two sites with vegetation and at two sites without vegetation, for a total of twelve trials. The results were then compiled to show the difference in water quality between the vegetated and non-vegetated areas.

Results:

For each tributary, the results of its vegetated sites in both the upper and lower reaches were averaged, so as to get one number for turbidity and  D.O. The upper and lower reaches of the non-vegetated areas were also averaged. The outcome of these tests can be seen in (Table 1).

The vegetated locations exhibited, on average, less turbidity and higher dissolved oxygen levels. While the non- vegetated areas had more turbidity and lower oxygen content.

Conclusions:

By reintroducing terrestrial environments to channelized streams early, water quality and overall stream health may be improved for the entire watershed. A better understanding of the overall difference between the vegetated and non-vegetated sites can be gathered by generating an image, which graphically illustrates the variation in water quality. This can be seen by using bar graphs which compare the averages obtained from the field data.

A comparison of the average turbidity of Milkhouse Creek, Moore Creek and Montlimar Canal in both their vegetated and non-vegetated areas is shown in (Figure 2). The vegetated areas, shown in green, averaged less overall turbidity than the non-vegetated areas, shown in black. This means that where vegetation occurred in the tributary, the water had less sediments suspended in the water column, and therefore, the water was clearer and cleaner.

The amount of dissolved oxygen/mL of water found in the vegetated and non-vegetated areas of each tributary is depicted in (Figure 3). The vegetated sites in green, exhibit a little bit higher dissolved oxygen content than the non-vegetated sites in black. This higher dissolved oxygen creates a healthy environment for fish and other organisms. These creatures, in turn, contribute to the nutrient load and help keep the water clear of algae. Overall, there was only a slight difference in the D.O. for the vegetated and non-vegetated sites. This may be because vegetation only plays a partial role in determining the oxygen content of the water. A higher concentration of dissolved oxygen is not only found where vegetation occurs in the streambed or along the banks, but occurs where water is moving and flowing.

Each water quality test performed in the abovementioned tributaries resulted in the anticipated outcome. For the most part, where vegetation occurs in the streambed, water quality is improved.

The results of this study will help to provide some evidence supporting the necessity for vegetation to grow freely along the banks and within the streambeds of the Dog River Watershed’s tributaries. Perhaps this study and future studies like it will prompt the city of Mobile to allow vegetation to reestablish itself in channelized streams, guaranteeing long-term health of the watershed. The DRCR could also benefit from this research by being the first environmental group to begin vegetation reintroduction projects into the watershed aiding in its overall improvement.

References:

Dog River Clearwater Revival website. http://www.southalabama.edu/geography/fearn/DRCR.htm

Hupp, Cliff R.. Ecology. “Riparian vegetation recovery patterns following stream channelization: A geomorphic perspective.” August 1992, Vol. 73, No. 4, pp. 1209-1226. Search Engine: JSTOR Accessed: 2/27/04

Vernon, Jacquelyn, A. “The Importance of Swamps and the Effects of their Destruction.” 2002. University of South Alabama, Department of Earth Sciences. http://www.southalabama.edu/geography/fearn/480page/02Vernon/Vernon.htm Accessed: 3/24/04

Moore, Dan R. and Richardson, John S. Canadian Journal of Forest Research. “Progress towards understanding the structure, function and ecological significance of small stream channels and their riparian zones.” August 2003, Vol. 33 Issue 8, p1349, 3p. Search Engine: EBSCO Accessed: 2/28/04

“Species are in danger? Reports puts Alabama at the top of the nation in number of aquatic animals imperiled by pollution.” Mobile Register. April 30, 1996. Search Engine: University Library, Mobile Register Archives. Accessed: 2/25/04

“West Mobile’s muddy waters.” Mobile Register. May 17, 1998, Edition AM, Section A, Page 1. Search Engine: University Library, Mobile Register Archives. Accessed: 2/25/04