Jessica D'Ambrosio

Effects of geomorphology, habitat, and spatial location on fish assemblages in a watershed in Ohio, USA

In this paper, we evaluate relationships between in-stream habitat, water chemistry, spatial distribution within a predominantly agricultural Midwestern watershed and geomorphic features and fish assemblage attributes and abundances. Our specific objectives were to: (1) identify and quantify key environmental variables at reach and system wide (watershed) scales; and (2) evaluate the relative influence of those environmental factors in structuring and explaining fish assemblage attributes at reach scales to help prioritize stream monitoring efforts and better incorporate all factors that influence aquatic biology in watershed management programs. The original combined data set consisted of 31 variables measured at 32 sites, which was reduced to 9 variables through correlation and linear regression analysis: stream order, percent wooded riparian zone, drainage area, in-stream cover quality, substrate quality, gradient, cross-sectional area, width of the flood prone area, and average substrate size. Canonical correspondence analysis (CCA) and variance partitioning were used to relate environmental variables to fish species abundance and assemblage attributes. Fish assemblages and abundances were explained best by stream size, gradient, substrate size and quality, and percent wooded riparian zone. Further data are needed to investigate why water chemistry variables had insignificant relationships with IBI scores. Results suggest that more quantifiable variables and consideration of spatial location of a stream reach within a watershed system should be standard data incorporated into stream monitoring programs to identify impairments that, while biologically limiting, are not fully captured or elucidated using current bioassessment methods.

A decade later: the establishment, channel evolution, and stability of innovative two-stage agricultural ditches in the midwest region of the united states

Much of the landscape in the Midwest region of the United States has been converted to agricultural use and with this conversion has come greatly altered hydrologic functioning. The elimination of wetland storage and installation of subsurface drainage systems and agricultural ditches has caused water to drain from agricultural watersheds at greatly accelerated rates. In some cases, these hydrologic alterations have led to severe water quality problems, including stream bank erosion, sedimentation, and inadequate processing of nutrients, each of which pose dire consequences for aquatic biota. Research by the authors has led to the modification of some trapezoidal agricultural ditches to two-stage geometries that are sized based on geomorphic concepts. A procedure for sizing these systems has been developed by the authors. Most of these innovative systems are located in Indiana, Michigan and Ohio. The main objective of the paper is to present details on how these systems have evolved since construction. The paper addresses issues that require further consideration. Channel evolution, determined by the assessment and analysis of physical condition, includes tracking changes in form by repeated surveys of channel dimension, pattern and profile. Pre-construction and post-construction properties are compared. Channel dimensions are also compared to regional curves. Analysis will include computing the hydrology and hydraulics for the range of recurrence intervals using the computer simulation models HEC-HMS, HEC GeoHMS, and HEC-RAS. All systems that have been studied have been stable, exhibited small adjustments on the constructed floodplains (benches), and have required little or no maintenance.

BEYOND THE FIELD: A LOOK AT AGRICULTURAL DITCH FLOODPLAINS AS A WATER QUALITY BMP

Agricultural fields, subsurface drainage, and a network of modified headwater systems dominate the landscape in the Midwest region of the United States. These systems are often the main conduits that export sediment and nutrients downstream, but very little is known about how these systems function. While most agricultural Best Management Practices (BMPs) focus on landscape measures to reduce sediment loss and water pollution little research has been performed on in-stream processes and channel system floodplains as agricultural BMPs. An important step in quantifying nutrient reduction capacities on floodplains is to predict discharges occurring on floodplains. This paper evaluates methods to predict the recurrence interval of discharges at ungaged sites and the annual exceedances of different discharge thresholds. Also presented is a study to evaluate the benefits of modifying channels to two-stage geometries that provide connection to floodplains and more bank storage. Preliminary results indicate that benches (small floodplains) should be located at elevations associated with about 25-35% of the 2-year discharge and they will usually flood about 10-60 days annually. Nitrate-N removal, in systems with flooded width ratios of 4 to 5 times the bankfull width, might be 5-20% of exports from fields if the treatment area (surface of the benches and inset channel) is about 1% of the watershed area. Results of this study are being used to develop a tool that quantifies the reduction in nitrate exports associated with different floodplain geometries and linear extent of the floodplains.