Siltation and the Cahaba River

(adapted from Don Elder, Cahaba River Newsletter, July/August 1991)

The central portion of Alabama holds some of our state’s and the nation’s most strikingly beautiful and biologically significant natural treasures.  Rolling, wooded hills, lush, verdant valleys, and countless rivers, creeks, lakes and streams combine to create an environment that supports a stunning diversity of life.  Ironically, however, some the same natural forces that helped give rise to many of these natural treasures also work to place them at particular risk today.

The Upper Cahaba Basin lies in a very complex geological area.  It features many very steep slopes, particularly around the Cahaba and its tributaries.  There are many different soil types found in the basin, and many of these are classified as “highly erodible”.  Rainfall amounts in the area are high, and there are usually several intense storms each year. Alabama’s highest annual runoff amounts are centered on Birmingham.  This high runoff rate is a combination of rainfall and the amount of impervious surface associated with urbanization.

The Upper Cahaba Basin is also the scene of some of the most rapid urban growth in Alabama today.  Many types of land use can lead to high levels of siltation in streams, but urban land uses--and in particular  urban construction sites and the impervious surfaces they create—generally produce higher rates of erosion than any other land use.

As a result of these factors, the potential for erosion and siltation in the Upper Cahaba Basin is among the highest in the nation.  In fact, many who know the river place siltation at the top of the list of the Cahaba’s problems. 

What is siltation, and why is it harmful?

Siltation is the settling of particles of soil and earth into the bed of a stream.  Certain amounts of siltation occur naturally, and are necessary to the ecology of a stream.  Some areas actually suffer from too little siltation today. Portions of the Mississippi River Delta, for instance, are disappearing because dams have altered the Mississippi’s natural function of delivering sediment to the mouth of that Great River.  As a result, the ecology of the Delta, and the unique social and economic structures dependent on it, are imminently threatened. 

But far too many rivers suffer from the opposite problem.  Too much silt can easily overwhelm a river, altering its physical characteristics, diminishing its beauty, and making it impossible for aquatic ecosystems to adapt to the pace of human-induced change.  Direct impacts of excessive siltation can include covering of shallow rocky areas with sediment and in-filling of deep pools.  These changes in habitat smother spawning sites, choke-out spaces where aquatic insects and mollusks live, and otherwise alter the natural places were these creatures have survived for thousands of years. 

There are other less direct, but equally insidious impacts of excessive sedimentation.  Silt deposited in deep pools can be stirred up repeatedly during high flow events, chronically increasing the turbidity (muddiness) of the river.  In this way, a single erosion and sedimentation event can continue to cause problems in the Cahaba River literally for decades. 

Turbidity can inhibit the process of photosynthesis.  Organic matter and nutrients contained in soil can rob a stream of oxygen.  Toxicants such as heavy metals (including lead, zinc, copper, and mercury) and hydrocarbons often occur in high amounts in urban runoff.  While all of these toxicants can be harmful in and of themselves, they can be even more harmful when combined with other pollutants present in streams.

What are the sources of erosion and siltation in the Cahaba Basin?

The Alabama Department of Environmental Management (ADEM) has identified five primary sources of siltation in our state:

• agricultural runoff,
• resource extraction,
• silviculture (forestry),
• channel modification,
• and urban runoff. 

The Cahaba receives siltation from all of these sources.

There are extensive agricultural and silvicultural activities in the lower Cahaba Basin.  We are currently working with the U.S. Fish & Wildlife Service and a local cattleman to fence cattle off the river in Bibb County.  Various types of resource extraction, including historical coalbed methane drilling, have contributed large amounts of silt to the river, the impacts of which continue today.  We are hopeful that recent coalbed methane drilling in the Cahaba River Basin has been undertaken by a group that will do a better job of managing their potential erosion impacts. 

While silviculture does not contribute cause extensive siltation while the trees are growing, we are concerned about erosion and sedimentation that follows tree-harvesting activities.  While the large corporate forestry operations generally do a careful job, smaller operators too often cut corners, leaving large areas with no sediment and erosion control and little or no buffer adjacent to streams.  Even the corporate forestry operations are not without impacts on nearby streams.  Questions regarding the biodiversity impacts of extensive clearcutting are another issue that we will not explore here.

Although there have been relatively few physical alterations to the Cahaba River main channel, there have been numerous instances where tributaries have been dredged, channelized, or dammed.  Many of these alterations have been done by local governments in misguided attempts to manage flooding problems.  While these efforts may bring some local relief to a localized problem, they generally worsen downstream flooding problems, usually for constituents of a different local government.

ADEM’s last category, “urban runoff” includes a complex set of problems that produce a variety of pollutants and conditions that degrade the Cahaba River.  In terms of magnitude, we believe urban runoff, as described in the following section, accounts for the vast majority of sediment being delivered to the Cahaba. 

Why is urban runoff such a large source of siltation?

In an extensive study on sediment problems in Shades Creek, the U.S. E.P.A. has distinguished between two significant sources of sediment.  They describe “upland processes” and “instream processes”. 

“Upland processes” refer to what we generally understand as a source for sedimentation to streams.  Construction sites have been estimated to comprise about 0.007% of the land surface in the nation at a given time.  However, sediment leaving developing subdivision averages about 25 to 30 tons per acre of development per year.  This rate is about 250 times higher than the 0.11 ton/acre/year erosion rate that has been set for the Cahaba River as a protective target erosion rate.  It is obvious from this and from our first-hand experience, that construction site sediment runoff can be an extremely significant source of sedimetation to the Cahaba River.

“Instream processes” are a subtler complex of problems that have been shown, at least for Shades Creek, to be a greater source of sedimentation than upland processes.  Instream processes refer to the bank erosion, stream-bed scouring, and bank slumping processes that are presently reshaping the Cahaba River channel shape.  To understand why this stream channel reshaping process is occurring, we need to understand the repercussions of urbanization and the increasing amounts of impervious surface that accompany urbanization.

Urbanization alters the landscape by increasing the percentage of land under pavement and structures that cause rainfall to runoff rapidly.

  As rainfall runs off rapidly, it is delivered to our local streams more rapidly.  Streets and landscaped areas are usually engineered to discourage accumulation of standing water.  These engineering solutions to flooding problems increase both the runoff rate and the runoff volume delivered to our streams.

Increased rainfall runoff volumes and rates each have negative effects on stream health.  The first is to diminish the amount of water that soaks into the ground and which subsequently maintains minimum flows in our streams.  As urbanization increases, experience has shown over and over that minimum flows in urban streams diminish markedly.  Without adequate minimum flow, aquatic wildlife simply cannot survive the altered low-flow regime because it is so different from what they have experienced for thousands of years.

The second negative effect resulting from higher runoff rates is that peak flows in streams are dramatically increased.  Instead of spreading out the delivery of stormwater runoff over time, our parking areas, rooftops, roads, and the other impervious surfaces we build into the urban landscape join with our system of storm gutters to deliver nearly all our rainfall to a local stream almost instantly.  The result is markedly higher peak flow volumes and higher flood heights than would otherwise occur without urbanization.  Streams adapt to those increased peak flows by modifying the stream channel.  Stream bed scouring, bank erosion, and bank slumping are all adaptations that the physical laws of river hydrology exact upon an urbanizing watershed. We are beginning to recognize that these stream hydrology changes, that are traceable back to urban planning decisions, are the root of perhaps the majority of the sediment problems in the Cahaba River.

(Please look for additional text at this location to follow soon.)