Sonntag, 29. März 2009

Beaver Basins

Geologists normally doesn´t consider animals as a mayor factor in sculpturing earth surface and landscape – yes, there are exceptions, like corals that build up large reefs, but there what counts is number and time, not the single organism.

Single or periodically operating events causing geomorphological features and distinct sedimentary patterns are much more rappresented by a vulanic eruption or a landslide, yet even a storm or a flood, ... but mammals? But in fact, some animals can produce geomorphological response in a sedimentary or erosional system, not directly , but for example by damming up rivers.

The potential of beaver (Castor canadensis) damming to have a significant geomorphic impact on postglacial fluvial landscapes has long been suggested and used to explain observed broad, flat valley floors in the Colorado Front Range.

Sediment deposition measured in modern beaver ponds shows that aggradation of up to ~1 m is common within several years after damming, up to a maximum account of 2m, considered the medium high for a beaver dam, that can last up to 46 years.

Drained beaver pond in Allegany State Park (from Wikipedia)

Larger depositions are therefore considered representing various beaver-dam generations, but until now very few studies were focused on this possibility. Beaver dam abandonment often results in collapse of the dam, and new channel incision through accumulated pond sediments and upstream alluvium by the new formed river.

Beaver dams can exist only where there are beavers, so for example habitat preferences, food resources and competition control where beaver dams are built, and should thus influence the long-term geomorphic impact of beavers.
Geomorphic factors also strongly influence beaver dam distribution along mountain streams, to large or to steep river channels, or to strong curent limits the ability of the animals to maintain dams.

In a study conducted by PERSICO & MEYER in Yellowstone National Park, beaver dam deposits were mapped and dated to understand the impact of beavers on the geomorphology of rivers and wetlands.
Dated subfossil remains give direct evidence of beaver in the park since at least the middle Holocene, and a hughe population rise in the years before park establishment in 1872 and a decline until 1920 (maybe because of competition with elks).

Diagnostic characteristics for deposits of beaver ponds mapped were determined by detailed field analysis of texture, sedimentary structures, Munsell soil color chart, and organic content, emphasizing features that distinguish beaver-pond deposits from other fluvial sediments. The sediments are characteristic fine-grained, deeperwater pond sediments, ranging from clay loam to loam, with minor sandy units. Pebbles and cobbles are sometimes scattered within deposits. Organic content in pond deposits is typically high, because of the habits of the beaver to introduce abundant organic debris in the pond, also some larger wood pieces showed beaver-gnawed ends. Beaver pond deposits were identified on parts of all streams examined, and individual pond deposits, i.e., those with no break in vertical sequence, ranged from 0.2 to 1.2 m thickness.

Incised-channel exposure on middle Elk Creek (Yellowstone National Park) showing 14C-dated beaver-pond deposits separated by oxidized gravel (~1.1–1.5 m depth).
Note stratigraphic position and age of ~1120 cal yr BP age from a beaver-gnawed Douglas-fir (Pseudotsuga) stump in growth position. The beaver-pond sediment shows lamination and contain organic-rich layers, pebbles, coarse to fine sand, and silt (from PERSICO, L. & MEYER, G. -2009, copyrigth 2008 University of Washington).

The chronological data, ranging between 7000 and 50 years seems to show a weak correlation of beaver dam sediments with cooler, and effectively wetter, climate phases, like the little Ice Age (~650–100 cal yr).The concentration of dated pond deposits in the last 4000 yr, essentially Neoglacial time, may also partly reflect a generally cooler climate and more reliable streamflows.
Historical climate records in the Yellowstone area show a trend toward a warmer and generally drier climate from 1895–1990 that has continued to the present, so some authors inferred that drought and reduced streamflows, especially in the 1930s, were partly responsible for the marked post-1920s decline in the beaver populations.

The amount of stream aggradation that can be attributed to beaver ponds is small, 2m for a single beaver pond, up to 2,5m for depositions with intervals between (with ages of deposition ranging between ~7000, 2600, 900, 400, and 50 yr mean ages).
Also, glacial erratic boulders along several rivers have not been buried by beaver-pond or fluvial deposits, indicating little net postglacial aggradation.

This results relative the importance of beavers as geomorphologic factor, that contribute only small amounts of sediment aggradation in river systems, nevertheless they play a role in a local context. The presence and the extent of the population of beavers is also influenced by ecological factors, like food ressources and competition, that themselves depend of the overall climate.

PERSICO, L. & MEYER, G. (2009): Holocene beaver damming, fluvial geomorphology, and climate in Yellowstone National Park, Wyoming. Quaternary Research. IN PRESS


Steve Gough hat gesagt…

Thanks for this enlightening post and your fascinating work. I've been amazed to see beavers build persistent dams in high energy, high coarse bedload streams in the Ozarks of Missouri, and have little doubt they had profound pre-Euro settlement effects on many of the streams I've worked on the the Midwest (USA). Your work will be very useful.

Anonym hat gesagt…

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