Indiana Biological Survey

Division of Fishes Projects

Great Rivers Habitat

Electrofishing on the Ohio River

Large-floodplain rivers (hereafter called great rivers) are distinctive in terms of their ecological operation and how humans have modified them. River components, including catchments, are physically and biologically connected along longitudinal, lateral, and vertical dimensions. Great rivers are subject to a variety of stressors, including impoundments that alter the flow regimes of water and sediments, pollution and land use practices that alter water quality and temperature, and intensive agriculture and wetlands reclamation that interrupt the connectivity of the floodplain and its associated wetlands and thereby disrupt energy flow. In great rivers, the disruption of the natural hydrologic and sediment regimes is evident in channelization, impoundment by dams, inundation and embayment of backwaters and tributaries, isolation and loss of wetlands, water withdrawal for irrigation and industrial uses, and excessive loading of fine sediment via land use in their catchments. Flow regulation has cascading effects on all aspects of the ecological structure and function of rivers, including altered sediment transport and temperature regimes, reduced production, fewer native species, and more nonnative species. As such, assessments of biological integrity for large rivers should indicate substantial impairment from the cumulative stressors of great-river basins.

Great rivers are also distinctive in the difficulties associated with assessing their biotic condition. Foremost among these are their size and the spatial scales over which habitat patches and biota are distributed. Scale has important implications for defining reference conditions and sampling biotic assemblages. Unlike smaller waterbodies, which are typically replicated across a given region, large rivers are typically unique, at least within the jurisdiction of a typical (e.g., state or province) management agency. This lack of comparable replicates severely limits the development of region-specific reference conditions, which commonly provide a basis for biotic assessments, and force a disproportionate reliance on historical accounts and expert judgment to define assessment benchmarks. This difficulty is exacerbated by the virtual absence of only slightly modified reaches from most large rivers; thus, even pseudoreplicate reference reaches are largely unavailable for comparison. Consequently, unless historical accounts are very explicit, which is rare, attributing observed patterns of variation (physicochemical or biological) to natural as opposed to anthropogenic sources might be arbitrary. Nevertheless, biological benchmarks can be defined on the basis of a general understanding of the ecology of riverine species and historical faunal conditions and by comparing the assemblage structure and function at anthropogenically impacted sites with those from relatively unimpacted sites. As such, they can substantially improve environmental assessments of large rivers.

The Ohio River begins at the confluence of the Monongahela and Allegheny Rivers and flows southwesterly for 1578 km through six states into the Mississippi River. The Ohio River crosses four ecoregions (the Western Allegheny Plateau, Interior Plateau, Interior River Lowland, and Mississippi Alluvial Plain). Nearly 10% of the U.S. population, more than 25 million people, resides in the Ohio River basin. The Ohio River has over 600 permitted discharges to its waters under the National Pollution Discharge and Elimination System (NPDES), including ones from industry, power generating facilities, and municipalities. Between 1885 and 1927, the Ohio River was impounded by 50 low-head navigation dams. Currently, 20 high-lift dams provide a 2.75-m minimum depth for commercial navigation, which transports approximately 250 million tons of cargo annually.

Trautman (1981) relates accounts from early settlers along the Ohio River describing abundant shifting sandbars, sandbanks, rock and gravel bars, and bedrock and rock ledges as well as clean bottoms and clear water except during floods. Degradation of the Ohio River occurred initially as a result of logging, agriculture, mining, and sewage effluent. Water quality in the Ohio River declined between 1810 and 1960 as a result of deforestation, increased agricultural activities, and increases in mining, industrialization, and urban sprawl that led to increases in mean turbidity, total dissolved solids, chlorides, nitrates, and sulfates. Acid mine drainage resulted in degradation of the upper 161 km of the river before 1950. Pearson and Krumholz (1984) and Lowman (2000) documented the decline of pollution sensitive species and the dominance of pollution tolerant species.

Blue catfish Ictalurus furcatus	Paddlefish Polyodon spathula