Glenn E. Griffith

Total alkalinity of surface waters: a map of the upper midwest region of the United States

This map (see the inside back cover of this issue) illustrates the regional patterns of mean annual alkalinity of surface water in the northern portions of Minnesota, Wisconsin, and Michigan, USA. It provides a qualitative graphic overview of the relative potential sensitivity of surface waters to acidic input in the upper midwest portions of the United States. The map is based on data from approximately 14,000 lakes and streams and the apparent spatial associations between these data and macroscale watershed characteristics that are thought to affect alkalinity.

How Misapplication of the Hydrologic Unit Framework Diminishes the Meaning of Watersheds

Hydrologic units provide a convenient but problematic nationwide set of geographic polygons based on subjectively determined subdivisions of land surface areas at several hierarchical levels. The problem is that it is impossible to map watersheds, basins, or catchments of relatively equal size and cover the whole country. The hydrologic unit framework is in fact composed mostly of watersheds and pieces of watersheds. The pieces include units that drain to segments of streams, remnant areas, noncontributing areas, and coastal or frontal units that can include multiple watersheds draining to an ocean or large lake. Hence, half or more of the hydrologic units are not watersheds as the name of the framework “Watershed Boundary Dataset” implies. Nonetheless, hydrologic units and watersheds are commonly treated as synonymous, and this misapplication and misunderstanding can have some serious scientific and management consequences. We discuss some of the strengths and limitations of watersheds and hydrologic units as spatial frameworks. Using examples from the Northwest and Southeast United States, we explain how the misapplication of the hydrologic unit framework has altered the meaning of watersheds and can impair understanding associations between spatial geographic characteristics and surface water conditions.

Quantitative Soil Descriptions for Ecoregions of the United States

Researchers have defined and mapped ecological regions of the United States based on similar patterns of ecosystems such as deserts, forests, and croplands. These studies are useful in regional research, monitoring, and environmental management because data can be more readily extrapolated within the same ecoregion and to regions with similar characteristics. The description of ecoregions is largely holistic and qualitative. Conversely, quantitative information for soil are abundant and soil is an important ecosystem component related to many ecoregion properties. We used the nationwide State Soil Geographic database (STATSGO) to describe the soils of 84 Level III ecoregions in the United States. Among the 24 soil characteristics studied were texture, rock fragments, available water capacity, bulk density, and organic matter content. For each ecoregion we developed ranks to describe (i) its similarity to the U.S. average soil characteristics, (ii) the accuracy of predicting those characteristics, (iii) how well the soil map unit boundaries fit within ecoregion boundaries, (iv) the spatial relationship of soils across neighboring ecoregion boundaries, and (v) the homogeneity of texture-rock patterns. We present a national map of soil texture and rock fragments and five soil ranks for each ecoregion, and examine relationships between soils and other ecological components for selected ecoregions. Because soils relate to other ecosystem components such as vegetation, geology, and land use, the soil ranks complement and enrich the qualitative ecoregion descriptions. Similar analyses of physical or biological components of ecoregions will expand the understanding of the ecosystem patterns.

Regional patterns of total nitrogen concentrations in the National Rivers and Streams Assessment

Patterns of nitrogen (N) concentrations in streams sampled by the National Rivers and Streams Assessment (NRSA) were examined semiquantitatively to identify regional differences in stream N levels. The data were categorized and analyzed by watershed size classes to reveal patterns of the concentrations that are consistent with the spatial homogeneity in natural and anthropogenic characteristics associated with regional differences in N levels. Ecoregions and mapped information on human activities including agricultural practices were used to determine the resultant regions. Marked differences in N levels were found among the nine aggregations of ecoregions used to report the results of the NRSA. We identified distinct regional patterns of stream N concentrations within the reporting regions that are associated with the characteristics of specific Level III ecoregions, groups of Level III ecoregions, groups of Level IV ecoregions, certain geographic characteristics within ecoregions, and/or particular watershed size classes. We described each of these regions and illustrated their areal extent and median and range in N concentrations. Understanding the spatial variability of nutrient concentrations in flowing waters and the apparent contributions that human and nonhuman factors have on different sizes of streams and rivers is critical to the development of effective water quality assessment and management plans. This semi-quantitative analysis is also intended to identify areas within which more detailed quantitative work can be conducted to determine specific regional factors associated with variations in stream N concentrations.

Interpreting patterns of lake alkalinity in the Upper Midwest region, USA

ABSTRACT Patterns in alkalinity data for lakes and streams in Minnesota, Wisconsin, and Michigan were analyzed and mapped as a first step toward determining the potential sensitivity of surface waters to acidic deposition. The mapping was accomplished by: (1) assembling available alkalinity data on as many surface waters as possible, (2) plotting these data on relatively large-scale maps; and (3) analyzing the patterns of the 14,000 plotted values for spatial associations with other characteristics such as land use, geology, physiography, and hydrology to aid in extrapolation of the data. The level to which each factor influences alkalinity varied within the region. In northeast Minnesota and parts of upper Michigan, bedrock geology can influence surface water alkalinity. In other areas of the region, glacial drift is often very deep, and alkalinity concentrations are influenced by the composition of the drift, the lake hydrologic type, and the amount of groundwater contact. The lakes of lowest alkalinity...