JAWRA Blog

December 2011 article (early view): “Threats to Riparian Ecosystems in Western North America: An Analysis of Existing Literature,” by Boris Poff, Karen A. Koestner, Daniel G. Neary, and Victoria Henderson

This literature review does more than simply list publications. Judging from topics of papers, it looks at how threats to riparian ecosystems have been perceived in the literature.

A total of 453 journal articles, reports, books, and book chapters addressing threats to riparian ecosystems in western North America were analyzed to identify, quantify, and qualify the major threats to these ecosystems as represented in the existing literature. Publications were identified either as research, policy, literature review, historical comparison, or management papers. While the types of threats vary on spatial and temporal scales, some persist through decades in western North America. This analysis shows that grazing has been perceived as a dominant threat since the 1980s, but has been diminishing in the past decade, while invasive species, dams and, in recent years, climate change are increasingly represented in the literature as threats to riparian ecosystems in western North America.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

August 2010 Article: An Evaluation of Qualitative Indexes of Physical Habitat Applied to Agricultural Streams in Ten U.S. States, by Robert M. Hughes, Alan T. Herlihy, and Philip R. Kaufmann

The diversity of metrics and methods for assessing physical habitat condition confounds comparisons among practitioners. Rapid qualitative indicators and assessments typically have higher sampling error and unknown bias, but time-intensive indicators limit the number of sites that can be sampled, all else being equal. This article presents a comparison of several popular indexes and concludes with some interesting observations on the roles of indexes and the need for more research.

The authors surveyed 51 previously sampled stream sites located in regions of row-crop agriculture in Oregon, California, North Dakota, South Dakota, Nebraska, Iowa, Minnesota, Pennsylvania, Maryland, and West Virginia to evaluate the comparability of four indexes of physical habitat condition relative to each other. The four indexes were: rapid bioassessment protocol (RBP); qualitative habitat evaluation index (QHEI); stream visual assessment protocol (SVAP); and qualitative physical habitat index (QTPH).

Their conclusion: “Despite varying perspectives of what ‘’habitat’’ is and the use of different metrics, these four qualitative physical habitat assessment protocols produced similar results. This suggests that if the goal is a rough estimate of overall physical habitat quality, the three wholly qualitative protocols are just as adequate as one based on multiple quantitative measurements of physical habitat (QTPH), which requires much more field effort. However, the highly significant correlations between macroinvertebrate biotic index scores and quantitative substrate metrics suggest that quantitative physical habitat measures may offer greater explanatory power than qualitative habitat assessments.” You’ll want to read the whole article before acting on this, because there obviously are a lot of qualifications.

The authors also note that protocols, indexes, and scoring criteria to suit natural regional and local differences and differing professional perspectives and management objectives hinder making national or regional assessments of stream habitat conditions in a consistent manner.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

June 2010 Article: Stream Condition in Piedmont Streams with Restored Riparian Buffers in the Chesapeake Bay Watershed, by Leslie L. Orzetti, R. Christian Jones, and Robert F. Murphy.

Overall, buffer age was positively related to improved indices of stream habitat, water quality, and benthic invertebrate metrics, with a time scale of 10-15 years.

Riparian buffer restoration is a tool utilized to reduce the impact of nonpoint source pollution in flowing waters around the world. If implemented correctly, riparian buffers have the capacity to convert and/or store nutrients such as nitrogen and phosphorus. Much research has examined the benefits of these restored areas for water quality control and improvement, but little has been done to validate the long-term efficacy and associated time lags of these buffers in restoring water quality.

This study tested the efficacy of restored forest riparian buffers along streams in the Chesapeake Bay watershed by examining habitat, selected water quality variables, and benthic macroinvertebrate community metrics in 30 streams with buffers ranging from zero to greater than 50 years of age. Results showed that habitat, water quality, and benthic macroinvertebrate metrics generally improved with age of restored buffer. Habitat scores appeared to stabilize between 10 and 15 years of age. Results are consistent with the hypothesis that forest riparian buffers enhance instream habitat, water quality, and resulting benthic macroinvertebrate communities with noticeable improvements occurring within 5-10 years postrestoration, leading to conditions approaching those of long established buffers within 10-15 years of restoration.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

April 2010 Article: “ The Role of Riparian Vegetation in Protecting and Improving Chemical Water Quality in Streams,” by Michael G. Dosskey, Philippe Vidon, Noel P. Gurwick, Craig J. Allan, Tim P. Duval, and Richard Lowrance.

Figure 1 from paper

Most Featured Collections feature at least one literature review, and this comprehensive one lists 5 pages of citations on the subject. The authors’ emphasis is on the role that riparian vegetation plays in protecting streams from nonpoint source pollutants and in improving the quality of degraded stream water.

The authors go beyond merely listing recent publications. Their conclusions offer some valuable insights. “Despite a large body of research into water quality functions of riparian zones and the existence of large programs that promote restoration of permanent riparian vegetation in developed landscapes, there have been few direct studies of the responses of stream water chemistry to the loss of riparian vegetation and to its restoration. Our analysis suggests that the level and time frame of water quality improvement depends on the type of pollutant and the processes that act on it, site conditions that determine how important each process is, and the amount of degradation in these processes that occurred prior to restoration. Legacy effects of past vegetation can continue to influence water quality for many years or decades and control the potential level and timing of water quality improvement.”

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]

April 2010 Article: “Modeling Stream Shade: Riparian Buffer Height and Density as Important as Buffer Width,” by David R. DeWalle.

This article is a nice little exercise in applying basic physics and geometry to explore buffer zone shading characteristics of small streams. Using a path-length form of Beer’s law to represent the transmission of direct beam solar radiation through vegetation, the author calculated impacts of varying buffer zone height, width, and radiation extinction coefficients (surrogate for buffer density) on shading for E-W and N-S stream azimuths.

DeWalle’s model suggests that at least 80% shade on small streams up to 6-m wide can be achieved in mid-latitudes with relatively narrow 12-m wide buffers, regardless of stream azimuth, as long as buffers are tall and dense. Although wider buffers may have other benefits, he suggests greater emphasis should be placed on tall, dense buffers to maximize stream shading.

[Please note: I have quoted and paraphrased freely from the article, but the interpretation is my own!]