THE ECOLOGICAL ROLES OF THE CLEAR CREEK ECOSYSTEM

Robert W. McFarlane, Ph.D. McFarlane & Associates Houston, Texas April 1998

Executive Summary

Clear Creek and its stream-side vegetation comprise a rare, and rapidly disappearing, riparian ecosystem valuable to terrestrial organisms, in-stream finfishes and shellfishes, and the significant commercial and recreational fisheries of Galveston Bay, Clear Creek in its natural state provides valuable ecosystem services at no cost to taxpayers, such as flood control, surface and subsurface water storage, sediment retention, nutrient and contaminant removal, maintenance of water quality, and habitat for plants and both resident and migrant animals.  Channelization of Clear Creek for flood control purposes will, by design, reduce or remove virtually all beneficial aspects of this riparian ecosystem.

Introduction

The purpose of this report is to summarize the ecological roles of the Clear Creek ecosystem; to illustrate their importance to the surrounding uplands, clear Lake, and the Galveston Bay ecosystem; and to predict the resultant impact of the proposed channelization of Clear creek upon these adjoining ecosystems.

The land adjacent to a stream or river is often called a riparian zone or a riparian ecosystem.  The riparian zone is a characteristic association of soils, plants and animals within the 100-year floodplain of a stream¹.   Riarian ecosystems are maintained by high water tables and periodic flooding.  Riparian zones have a multitude of ecological functions including surface and subsurface water storage, sediment retention, nutrient and contaminant removal, and the maintenance of habitat for plants and animals¹.  Riparian zones suppress the undesirable effects of flooding, maintain water quality, and serve as centers of biological diversity.  There are continuous interactions between the riparian, aquatic and upland terrestrial ecosystems through exchanges of energy, nutrients, and species.  Riparian ecosystems are characterized by the combination of high species diversity, the density of organisms, and the productivity of the system.  These ecosystems receive seasonal or periodic pulses of water level that are delivered from overbank flows carrying nutrients and organic matter¹.

Riparian ecosystems have a linear form as a consequence of their proximity to rivers and streams.  The energy and material from the surrounding landscape converge and pass through riparian systems in much greater amounts than other wetlands.  Riparian systems are open systems.  They are functionally connected to upstream and downstream ecosystems and are laterally connected to upslope (upland) and downslope (aquatic) ecosystems².

The Hydrologic Role

Riparian ecosystems have a high water table because of their close proximity to the adjacent stream.  Their periodic, sometimes lengthy, inundation leads to the development of anaerobic conditions in the soil.  Thus plants must be adapted to both inundation by water and a lack of oxygen in the soil.  Flooding creates natural levees along the streambank as floodwaters spread out and lose velocity, allowing larger sediment particles to settle out.  These levees then retard the flow of water back into the stream as floodwaters recede, resulting in further sedimentation.  This provides a source of topsoil eroding from nearby uplands, and a source of moisture during drier periods, promoting the growth of trees and herbaceous vegetation.  As the leaves and stems fall to the ground seasonally, decomposition is aided by the moist conditions and abundant soil organisms.  The result is a rich organic soil, which frequently acts as a sponge to retain moisture.

This periodic flooding is necessary to maintain the riparian forest.  In return, the floodplain forest accommodates large volumes of water, reducing the peak flood, recharging the groundwater, and slowly feeding the water back to the stream system, minus that amount which evaporates, is transpired to the atmosphere by the riparian vegetation, or is absorbed as groundwater.  By creating meanders and oxbows, the stream further slows the downstream water flow.  Streamside vegetation shades the stream, reducing water temperatures during the warm season.

Because of its proximity to Clear Lake and Galveston Bay, Clear Creek has a salt wedge as a result of the density gradient.  This produces an upstream flow of salt or brackish water along the bottom of the creek at the same time that freshwater is flowing downstream at the surface.  The saltwater wedge is influenced by downstream flow and tidal action, and its upstream limit oscillates between the mouth of Chigger Creek and FM 528.  As a result, typical marine organisms can venture far upstream in the creek.

The Nutrient Role

The Clear Creek riparian ecosystem serves as both a sink and a source of nutrients.  It continually receives nutrients in precipitation runoff from the adjacent uplands which lie upslope.  It periodically receives nutrients from floodwaters which top the natural bankside levees, as sediments settle out.  It stores organic materials from leaf fall and remineralizes inorganic nutrients through the decomposer organisms on the forest floor.  As floodwaters recede from the forest, organic and inorganic nutrients, particulate materials, and larger detritus are carried downstream to Clear Lake and Galveston Bay.

It is important to differentiate between the nutrient cycling of the creek ecosystem compared to the lake and bay ecosystems.  This creek receives all of its nutrients from the terrestrial portions of the watershed (technically described as a heterotrophic ecosystem) where the photosynthesizing organisms are terrestrial plants.  The lake and the bay, which experience much slower water movement, are supported by two nutrient cycles.  One cycle involves phytoplankton which use direct sunlight for photosynthesis (an autotropic system) and the food chain includes zooplankton, small fishes, bigger fishes, etc.  The other cycle is based on the consumption of detritus (decomposing plant and animal material), much of which is imported from the streams and rivers discharging into the bay.  Many marine organisms, such as shrimp, crabs, catfish and mullet, are consumers of detritus³.

The numerous side bays and oxbows of Clear Creek play an important role in nutrient and detritus production.  The freshwater, intermediate, and brackish marshes, which are prominent in these locations, are important contributors of organic and inorganic nutrients, particulate matter and large detritus.  The different plant species die back at different times, decompose at difference rates, and together provide nutrients and detritus to the creek over an extended period of the year.  These sidebays and marshes are important nursery areas for postiarval and juvenile shrimp, crabs, and numerous fish species.  For this reason they are very important to the extensive fisheries of Galveston Bay.  In reality, the bay begins in the upstream headwaters of its creeks and rivers.

The Water Treatment Role

Riparian floodplains improve water quality by removing suspended solids, nutrients and contaminants.  Pollutants that are attached to clay particles precipitate with the sediment and remain in the forest soil layer for varying periods of time.  The forest vegetation removes soil particles eroding from the adjacent uplands.  This important role of riparian vegetation has been recognized across the nation, and steps to improve or replace riparian habitat are underway.  Improving fish habitat frequently begins by fencing cattle away from stream edges.

The Biodiversity Role

Some of the highest densities of breeding birds in North America are found in riparian habitats.  Streamside vegetation is very important in determining the structure and function of stream ecosystems.  A riparian forest provides habitat for nesting and food acquisition that is absent from the surrounding prairie (originally), agricultural land, and developed areas.  As in the stream banks slowly erode, trees topple into the stream, still attached to the bank, and create habitat for many stream organisms.  Algae and many small invertebrate animals attach to the trunk, branches and exposed roots.  In turn, these provide food and shelter to small fishes, which are fed upon by larger fishes, reptiles, birds and mammals.  These toppled trees, sometimes called snags, create habitat by slowing down the flow of water and providing eddies of backflow.  Fishes, and fishermen, are attracted to these habitats, as their attached hooks, lures and fishing line will attest.

Many freshwater fishes occur in the surface water while estuarine fishes are found in saltier water along the bottom.  Some estuarine organisms, like menhaden, mullet, hogchokers and blue crabs, penetrate far upstream into fresh water.  When the floodplain is under water, both freshwater and estuarine species spread out across the floodplain to feed on newly available forest floor organisms.  Large fishes which have grown up in isolated oxbow lakes are able to escape to the stream, while the supply of new small fishes in the lakes is replenished.

The fishes of Clear Creek and Clear Lake are both abundant and diverse.  Fifty-eight species of fish have been captured in Clear Lake.  The following table describes the number and quantity of fishes, which are impinged on the trash-removal screens for cooling water drawn into the HL&P Webster Generating Station in one year⁵.  The intake canal for this plant is located at Mile 7.8 on the lower reach of Clear Creek.  Most of the impinged fishes are small, for larger fish are able to escape the water velocity, which draws them into the screens.  The number of detritivores and bottom-feeding fish is striking.  These organisms are all dependent on detritus and food organisms carried downstream by the creek.  All of these estuarine finfishes and shellfishes had entered Clear Creek.

 The Environmental Impact of Stream Channelization

The channelization of Clear Creek will reduce or remove virtually all of the beneficial aspects of riparian habitat.  It is designed to do exactly that.  It will be built to accommodate a rear event.  Major flooding seldom occurs.  It happened in 1932, 1940, 1942, 1946, 1959, 1973, 1976, 1979 (twice), 1989, and 1994; or 11 times in 65 years.  It results from people building homes in an inappropriate area, the floodplain.

Channelization will shorten and straighten the stream.  This will result in more water moving downstream faster.  Less water will reach the floodplain.  Less water will infiltrate to become groundwater.  Less sediment, and fewer contaminants, will be deposited on the floodplain.  The lateral flow of water from adjacent uplands will be diverted by the new levees.  Less nutriment and detritus will be carried downstream to nursery areas, Clear Lake and the bay.  Important microbial decomposition processes that depend on in-stream travel time will be shortened.  Since the Texas Department of health has issued a fish consumption advisory for Clear Creek due to contamination from the Brio superfund site on Mud Gully, any contaminants stirred up during channelization will quickly reach Clear Lake and Galveston Bay.

Channelization will lower the stream.  During low stream flow (that is to say, most of the time) the water level will be at a lower elevation.  This will dewater the adjacent floodplain by lowering the water table.  Existing floodplain vegetation will be greatly affected.  Since the entrance to oxbow lakes will be lowered to the new channel depth, these lakes will also dewater during low flow periods.

Channelization will widen the stream.  This will necessarily remove much of the floodplain vegetation.  Even if the impact is restricted to just one-half of the floodplain, (one side) it will decrease the riparian habitat by half.  By greatly reducing the streamside trees and vegetation, and the shade they create, it will increase water temperature during the summer, and increase the amount of light reaching the stream at all times of the year.  Widening the stream will necessarily decrease the depth of water during normal flow, further increasing water temperature and light exposure, fostering the growth of noxious algae.

Channelization will remove in-stream snags, which are the prime fish habitat.  In fact, channelization removes the stream, converting it into a ditch.  Channelization will fragment the existing riparian habitat.  Since this habitat is already rare, and rapidly disappearing, organisms which are dependent upon this habitat will find it more difficult to locate, and to move between fragments, which will be farther apart.  Channelization is designed to narrow the floodplain, making more land available for development, which indirectly will further reduce and fragment the riparian ecosystem.

Channelization will not achieve its stated objectives of enhancement of fish and wildlife resource (attract more and varied species), recreation, water quality, and preservation and protection of natural and cultural resources for public education and historical appreciation⁶.  In fact, channelization will reduce fish and wildlife diversity, recreation and water quality, and destroy the natural resources of the riparian ecosystem.

References Cited