While designing this experiment, we asked ourselves several questions concerning the Northfield Dam in the Cannon River and its potential effects on mussel populations downstream. Reviewing various literature sources, we focussed on the mass of sediment above the dam and the possible effects of releasing this build-up, were the dam to be removed.
We addressed the following questions:
How does sediment in a river system affect mussels?
Human activity has greatly altered the water quality of the Cannon river and many other waterways throughout North America in the past two centuries. Activities such as row cropping and widespread construction have lead to vast amounts of erosion which greatly affect the turbity and overall quality of a river's water (Robbins, 1996). A large number of mussel species have gone extinct or have suffered severe decline in numbers in the past century. A principle cause of such population loss was over-harvesting of mussels for the pearl industry in the U.S. that began in the late 1800's and continued until World War two (Sparks). Another contributer to the reduction in mussel populations since then has been extreme buildup of suspended and deposited sediment in America's river systems, including the Cannon (Williams et al., 1993).
What are the ecological impacts of dam removal?
A dam creates a pool of slowly moving water in the river immediately upstream. Sediment particles in the river slow in this pool and are deposited along the riverbed, forming a large plume just behind the dam. A prime concern when removing a dam is the release of this sediment. Depending on the span of time during which the sediment has accumulated, this plume could cause problems downstream by significantly increasing turbitity. Such a drastic change in water quality could jeopardize the survival of organisms that filter-feed or rely on light to penetrate the water (Alexander et al,1986).
Does sediment build-up affect mussel growth?
Growth rate is greater in mussels living in sand/gravel sediment than those found in conditions of higher silt concentrations (Kat 1982). Kat attributed the mussels' higher growth in less turbid environs to silt occluding their siphons, interfering with the mussel's ability to filter feed the surrounding waters. Similarly, mussels located in waters of heavy siltation and erosion, such as downstream from dredging areas also show reduced growth rates (Marking and Bills, 1979). However, at least one species, Lampsilis radiata siliquoidea, prefers muddy substrates over gravel or sand surfaces (Baily 1989). Baily's study indicates variation in substrate preference among species of mussels.
In some cases, mussels may be buried by silt build-up. One study showed that mussels buried up to 15 cm under silt have a 50% chance of emerging over 6 hours before death by (Marking and Bills, 1979). Some species such as the Lampsilis ventricosa, were more adept at emergence. The study attributed the superior motility of L. ventricosa to its relatively large, broad foot. Others species cannot emerge from silt as effectively. Pleurobema sintoxia, for example, showed rates of 45% emergence at 10 cm. These findings indicate differences among mussel species in their susceptibility to being buried by silt.
After reviewing literature reviewing the effects of damming on freshwater mussels, we formulated questions for our study in the Cannon River. We began by estimating the amount of silt immediately upstream of the dam- the amount that would presumably be released upon removal of the dam.
(Overview of sediment build-up)
We also set out to perform a survey of mussel populations in the area downstream from the dam (Click here for photo of the area). Because of their proximity to the dam, these populations would be the most immediately affected were the dam to be removed. Using our findings and data from other studies, we then set out to estimate how the removal of the Northfield dam would impact mussels species immediately downstream.