Because streams and rivers are so fundamentally linked to the watersheds that surround them, most of the threats to terrestrial habitat systems also threaten streams. If the watershed and habitat systems within them are fully functioning and intact, the stream is more likely to be healthy, as it is a reflection of that watershed. Of course, in-stream impacts, such as channel dredging, channelization, and damming, also have direct and severe impacts on aquatic systems.
Construction activities without effective erosion control can cause sedimentation in streams. In developed urban areas, impervious surfaces like roads, buildings, rooftops, etc. can have the opposite effect by not allowing enough sediment into streams, especially when the channels themselves are put through pipes, culverts or lined in concrete. This can cause excessive velocities that erode the stream channel and degrade stream habitat. Frequent urban water quality problems include increased stream temperatures from impervious surfaces, lack of riparian buffers; and pollutants from vehicles, yards, and municipal sewage overflows, etc.
Overgrazing can increase erosion and run-off into stream channels which can increase sedimentation that creates turbidity and fills interstitial spaces that are critical habitat to benthic organisms. Excess nitrification from manure that enters streams can cause algae blooms and decrease water quality. Certain row cropping practices can also be detrimental to streams by allowing exposed soil to erode off fields, causing stream sedimentation. Fertilizer and chemical run-off can also negatively affect water quality. Tiling practices change the delivery rate of water to streams by condensing water into underground tubes that are often piped directly to a stream, enter at high velocities, and can erode the stream channel. Cumulatively, throughout a watershed, these and other practices can have a dramatic effect on habitat, water quality, and biota in a stream system.
Streams rely on their watershed connections that run horizontally into the riparian area and floodplains, longitudinally up and down channels, and vertically between the channel bed and the water table. Common causes of horizontal connectivity loss occur in floodplains and riparian areas when development or levees encroach on floodplains, side channels and oxbows are filled in or cut off, or riparian vegetation is removed or altered. Alterations of natural ecological flow regimes from industry, municipal or agricultural uses, or downstream of large dams, and many other causes can also cause this loss.
Longitudinal connectivity is critical for fulfilling migration requirements, genetic dispersal, and habitat utilization of many aquatic organisms. Longitudinal barriers are created by limiting the movement of organisms physically or behaviorally; dams, poorly designed road crossings (e.g., slab concrete crossing), and culverts are common examples. Large reservoirs and the cumulative effects of small ponds have altered hydrology, habitat, and aquatic species throughout the state as well.
Stream Habitat Destruction
In-channel activities, such as channelization, improper mining activities, channel reaming, filling, burying or excessive armoring, and others can cause localized and system-wide losses to stream habitat. Deforestation and the loss of an adequate riparian corridor throughout much of the state, ongoing since settlement, have altered stream hydrology and habitat and energy cycles. In addition, the loss and lack of wooded stream corridors deprives stream channels of large woody debris which maintains and creates various habitat types throughout the channel network and is a critical component of the food chain for invertebrate and vertebrate species.
Beyond ecological concerns, aquatic invasive species have tremendous impact, on local, state, and federal economies, impacting aquatic industries like water treatment, commercial and sport fisheries, recreational boating, etc. Terrestrial invasives are no different, and combined, in the United States alone, these invasive species amount to hundreds of billions of dollars per year to manage.
Like terrestrial habitat systems, aquatic systems are extremely vulnerable to the effects of invasive species, especially due to the high connectivity of most aquatic systems. Connectivity can be both a benefit and a detriment to a system. Connectivity benefits native species by minimizing habitat fragmentation and allowing species and genetic diversity and distribution, but it also allows for the rapid population expansion and distribution of invasive species. Some of the most well-known aquatic invasive species in Missouri include zebra mussels (Dreissena polymorpha), quagga mussels (Dreissena bugensis), invasive carp including bighead and silver carp (Hypophthalmichthys nobilis and H. molitrix), hydrilla (Hydrilla verticillata), and didymo, or "rock snot" (Didymosphenia geminata).
These invasives are highly competitive with native species with impacts that can include direct competition for food, predation, displacement, smothering or shading, disease introduction, and potentially, interbreeding. Any one or combination of these factors can lead to upsetting the delicate balance of native aquatic ecosystems.