K. D. Johnson raises an excellent point about the impact of beavers on pre-settlement streams. We did not, however, overlook the impact of these animals. In Walter and Merritts [(1), p. 302] we state: "This regional network of small streams and low, vegetated islands within the flood zone was probably impacted heavily by beaver dams and small ponds…" [c.f., (2)]. We intended to say more on this subject, but other topics took precedence. We welcome this opportunity to elaborate on the geomorphic impacts of beaver.
The online article referred to by Johnson (3) is a cogent summary of some of the classic work on modern beaver populations. This article and references cited therein demonstrate that valleys impacted by beaver are quite similar to our description of pre-settlement waterways in the mid-Atlantic region of the United States, as based on geological, geochemical, and geomorphological analyses of the ubiquitous, buried pre-settlement wetland (hydric soil) (1).
Regarding observations of modern beaver ecology, Pollack et al. (4) infer that "beaver dams created stream systems with slow, deep water and floodplain wetlands dominated by emergent vegetation and shrubs." Hemenway states: "A natural North American stream is not a single, deeply eroded gully, but a series of broad pools… stitched together by short stretches of shallow, braided channels" (3). Note the term "braided" has a specific meaning in geomorphology and is associated with high-energy streams with high sediment loads; the systems as described by us and presumably by Hemenway—which are low gradient streams with low pre-settlement sediment loads—are more appropriately referred to as "anabranching" (5).
Compare these statements to our description of pre-settlement stream systems in the mid-Atlantic: "Valley bottoms along eastern streams were characterized by laterally extensive, wetland-dominated systems of forested meadows with stable vegetated islands and multiple small channels during the Holocene… In particular, logjams blocked channels and led to the formation of side channels and floodplain sloughs, producing multiple anabranching channels and riverine wetlands that are in stark contrast to the large, single channels that exist in these streams today" (1). This convergence of ecological and geological perspectives underscores the important link between beaver activity and the shape, function, and stability of stream systems.
There is substantial added value to beaver habitats in streams. Beaver ponds trap sediments, create organic-rich wetlands, store carbon, filter nutrients, increase biodiversity, reduce flow velocities, and recharge groundwater (2, 4, 6–8).
Prior to European settlement, beaver were abundant and had considerable geomorphic impact on stream valleys in the mid-Atlantic Piedmont, but there was essentially no sediment being shed from the forested upland hill slopes [long-term erosion rates of < 0.006 cm/yr, (9, 10)] until European settlers began clearing the land in the late 17th and early 18th centuries (11, 12). Without that sediment supply, there was little to fill in beaver ponds except organic matter. In mid-Atlantic stream valleys, the pre-settlement hydric soil averages about 30 cm thick, and formed over the last ~10,000 years (1). In contrast, post-settlement alluvium averages about 2.5 m thick and formed within the last 300 years over a time span of about 150 years. The pre-settlement sediment accumulation rates in valley bottoms were low (~0.003 cm/yr) and consistent with the long-term geological erosion rates, compared with the unusually high post-settlement accumulation rates (~1.7 cm/yr).
The construction of numerous small beaver dams helped create the anabranching stream networks in the mid-Atlantic region during pre-settlement times, and beaver were an important factor in creating the pervasive wetlands that are now buried beneath thick stacks of post-settlement mud. Should beaver be reintroduced into mid-Atlantic streams, or can humans be as effective as beaver in engineering and adding value to streams?
Robert C. Walter and Dorothy J. Merritts
Department of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604, USA.
References
1. R. C. Walter, D. J. Merritts, Science 319, 299 (2008).
2. D. R. Butler, G. P. Malanson, Geomorphology 71, 48 (2005).
3. T. Hemenway, Permaculture Activist 47, Summer 2002; http://www.permacultureactivist.net/articles/Beavers.htm.
4. M. M. Pollock, M. Heim, D. Werner, Am. Fish Soc. Symp. 37, 213 (2003).
5. G. C. Nansen, J. C. Croke, Geomorphology, 4, 459 (1992).
6. R. L. Ives, Journal of Geomorphology 5, 191 (1942).
7. R. J. Naiman, C. A. Johnston, J. C. Kelley, BioScience 38, 753 (1998).
8. J. P. Wright, C. G. Jones, A. S. Flecker, Oecologia 132, 96 (2002).
9. M. J. Pavich, Appalachian Geomorphology, T. W. Gardner and W. D. Sevon, Eds. (Elsevier, Amsterdam, 1989), pp.181-196.
10. Hancock and Kirwan, Geology 35, 89 (2007).
11. G. S. Brush, Limnology and Oceanography 34, 1235 (1989).
12. W. B. Hilgartner, G. S. Brush, Holocene 16, 1 (2006).
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