Vol-1 Issue-1

date: 
Thu, 07/01/2010

 

SEDIMENT IN STREAMS

 "Sediment is far and away the major polluting agent in this country.  In terms of volume, it exceeds in importance all other sources of pollution combined: domestic, industrial, agricultural, and other wastes." (Satterlund 1972) In addition to its volume, sediment can be harmful to society in many ways.  Sediment can kill aquatic life in streams and rivers, it can increase the cost of treatment for water supplies, it can rob capacity from public water supply reservoirs and it can cost hundreds of millions of dollars to dredge from shipping channels in bays and harbors.  In other words, it affects all of us.

 While few would dispute the above statements, it is incredible that there is no program that effectively addresses this problem.  The Clean Water Act, which over the past twenty three years since its passage in 1972, has resulted in widespread and far reaching improvements to water quality does not even address sediment in any significant way.    Why is this? 

 The major reason is that sediment -- its sources and its effects-- is not well understood.  Sediment is a natural material present in all water bodies.  When it occurs in quantities or in forms that are not tolerated by aquatic life it does damage.  For example, sediment can eliminate habitat by filling in the natural gravels that occur in streams and rivers.  This habitat may be critical to fish that lay their eggs in gravel, and aquatic macroinvertebrates that spend part of their life cycle in these gravels.  When this happens it is obvious that it happened.  Where the sediment came from may not be obvious.  Did it come from uncontrolled construction activities, agricultural activities, forestry operations, or did it come from an eroding stream bank upstream of the problem site?  

 Many states have very good regulatory programs to control sediment from construction and mining activities; however, there are no programs that address all of the important sources of sediment.  Although there is widespread understanding that sediment is causing a lot of damage, there are no universally accepted means of measuring sediment or quantifying acute and chronic damage levels.  The focus of the clean water act has been on efforts to quantify concentrations of suspended sediment which can be harmful.  Suspended sediment can be measured and standard methods exist for this measurement. However, suspended sediment is not the only form of sediment that does damage.  Only the finest particles of sediment are suspended for significant periods of time.  What settles more quickly may be harmful but never gets measured.  A further complication in measuring sediment is the fact that much sediment moves as bedload in rivers in streams.  It is unseen and measurement or quantification of this form of sediment is seldom done. 

 There are no standard methods for measuring the effects of sediment on habitat and this is the area where a great deal of damage occurs to aquatic life.  To make matters more confusing, sediment is a universally occurring material in water bodies.  To try to treat it as a pollutant and regulate it as such really confounds the issue.  Its presence does not necessarily indicate pollution.  Its concentration for brief periods of time as suspended sediment (although measurable) is not easily related to either its potential for damage or any identifiable source.  No wonder then that conscientious legislators and regulators have been ineffective in developing a Apollution control program@ for sediment.

 Since sediment is ever present in waterways, we need to understand which processes and under what conditions sediment is a pollutant, or more important, does damage that we wish to eliminate.  The approach to water quality improvement in the clean water act amendments which was passed into law in 1972 was control pollutants.  The main sources focused upon in the law have been discharges from industrial operations and sewage treatment plants.  These were important elements and Americans have seen great water quality benefits as a result of the NPDES program and the construction grants program.  During this time damage from sediment to waterways has continued unabated.

 Some recent efforts to measure bedload sediment have shown that when streams or rivers are in a state of disequilibrium that bedload sediment can be a significant portion of total sediment load.  More important, when sediment transport and sediment supply are not in a dynamic equilibrium the excess sediment from channel erosion can feed on itself and maintain conditions of excess fine sediment for decades.  The process is one in which excess sediment creates depositional features which accelerate bank erosion. Bank erosion generates more excess sediment which causes more bank erosion and on and on.  The resulting excess sediment load can virtually eliminate aquatic life while it persists.

 The problem is that rivers and streams have channels that are in equilibrium with the peak discharges from frequent high flows called bankfull or dominant discharges.  Land use changes can alter the hydrologic cycle by changing the amount of runoff that reaches streams during any given rainfall event.  If this peak discharge increases, as it does with increase in impervious area or loss of vegetation, then the channel must enlarge to establish a new equilibrium size to carry these greater flows.  This process of channel enlargement is what starts the process of accelerated adjustment.

 This condition is common in watersheds which have experienced significant land use changes.  It is important to be able to recognize this condition and to develop strategies for dealing with it if we are to restore and maintain the health of our rivers and streams.  The problem can be mitigated to a great extent with the appropriate storm water management.  However, many rivers and streams are already in a state of accelerated adjustment as a result of sediment disequilibrium and the only remedy at this stage is in-channel restoration work to reduce sediment supply and/or improve sediment transport.

 What is needed is a better understanding of the condition of disequilibrium in which sediment supply exceeds sediment transport.  There may be thresholds that are different for different streams.  There may be aquatic organisms which can tolerate differing quantities of sediment.