The effects of dams on river morphology and fluvial processes have become increasingly important to watershed management during recent decades. Managers now routinely assess the effects of dam operations on sediment transport processes downstream of dams to aid stream ecological processes, and issues related to dam removal are widely discussed by a variety of stakeholders (Graf, 2003). The geomorphic changes that arise downstream of dams have important effects on endangered species, riverine habitat, engineering structures, and a host of other important issues in water resources management (Petts, 1984).
There is an extensive literature documenting how dams change downstream channels (Williams and Wolman, 1984; Brandt, 2000; Fassnacht et al., 2003; Grant et al., 2003; Graf, 2005). However, these studies have tended to focus on larger dams rather than the much more abundant smaller dams, and field observations are not uniformly distributed geographically (Graf, 2005). Several authors have tried to develop generalized conceptual models to help predict the downstream effects of dams on rivers, but these efforts are in their infancy (Brandt, 2000; Schmidt and Wilcock, 2008). Our present understanding of the geomorphic influence of dams on downstream channels recognizes a welter of important controls that can provide widely varying geomorphic responses in individual cases (Grant et al., 2003).
The geomorphic effects of dams are important in Pennsylvania, Maryland, and surrounding states because this region is a hotbed of activity for dam removal (Bushaw-Newton et al., 2002). Understanding the potential channel changes that can arise after dams are removed often requires an assessment or understanding of existing dam impacts. The influence of dams on sediment budgets and fluvial sediment transport processes is of concern for a variety of efforts to restore channels and estuaries downstream of dams in the region (Walter et al., 2006). Despite the extensive interest, however, few studies have documented the geomorphic effects of dams in this region, though some regional assessments of the hydrologic impacts of dams are available (Magilligan and Nislow, 2001).
This study represents an initial effort to document the geomorphic influence of dams on downstream channels in Pennsylvania and Maryland. Because the majority of dams in the region are small dams (Poff and Hart, 2002), these provide the primary focus for our research.
The clearest means of documenting how dams influence channels downstream would be to initiate a monitoring program before dam construction, and to document how the channel responds through time after the dam is built. Because the initial surveys are unavailable (and also because the dams are already in place), we were forced to adopt a different approach.
In addition to field observations of the present channels below dams, we required a reasonable surrogate to represent conditions downstream before the dams were built. More precisely, we needed to locate a channel that had not been influenced by the construction of the dam, but whose underlying bedrock, levels of human disturbance, hydrologic regime, and sediment supply are similar to those of the downstream channel.
As a working hypothesis, we propose that the channel upstream of the impoundment provides the best available means to define geomorphic conditions downstream before dam construction (assuming that we can always carefully locate our sampling reaches above any influence of hydraulic backwater effects or deltaic sedimentation associated with the impounded channel downstream). This approach should be reasonable if the geomorphic character of the channel above the impoundment is similar to that below the dam, and if differences in controlling variables before dam removal can be scaled (if necessary) by simple measures such as drainage basin area. We address these conditions in detail below.
We selected 13 streams in Pennsylvania and two in Maryland (Figure 1). Twelve sites are located in the Piedmont Physiographic Province, while the other three sites are located in the Ridge and Valley Physiographic Province (Sevon, 2000). Pennsylvania has a humid continental climate, with temperatures usually ranging between −18°C and 38°C. Summers are generally warm, averaging between 20°C and 23°C. During the coldest months, temperatures average near the freezing point. Annual precipitation varies from 865 to 1320 mm (http://climate.met.psu.edu/data/IA/).
The streams of our study sites are primarily single-thread channels with coarse-grained bed material and forested riparian zones. Aside from the presence of dams, the study reaches have no engineering structures or other evidence of direct anthropogenic controls. The streams vary from first to fourth order (using the Strahler stream ordering scheme) (Strahler, 1957) (Table 1). Drainage areas range from less than 1 to about 300 km2 (Table 1).
|Dam Name||Stream||ID#||Dam Height (m)||Drainage Area (km2)||Stream Order||HRT (days)||Dam Age (years)|
|Core Cr.||Core Cr.||1||14.3||24.46||3||54.44||31|
|Fish Weir||Dismal Run||2||0.9||0.80||2||0.01||NA|
|Prettyboy Reservoir||Gunpowder R.||3||57.0||198.85||3||382.26||74|
|Haskins Dam||Tohickan Cr.||4||1.5||110.19||3||0.20||150+|
|Hickory Run Dam||Hickory Run||6||3.0||7.24||1||1.74||70+|
|Mensch Mill Dam||W. Br. Perkiomen Cr.||8||2.1||11.77||1||0.12||185|
|Woodbine Dam||Muddy Cr.||9||1.5||287.19||3||0.01||150+|
|Stametz Dam||Sand Spring Run||10||4.6||9.51||1||0.26||70+|
|Stimson Run Dam||Stimson Run||11||8.8||0.72||1||26.82||22|
|Unnamed||Tributary Middle Cr.||12||0.9||3.40||4||1.07||70+|
|Dr. Jackson Dam||Swamp Cr.||13||1.2||115.98||1||0.18||240|
|Lake Conewago||Conewago Cr.||14||5.5||3.94||1||14.43||88|
|Atkinson Dam||Winter’s Run, Otter Pt. Cr.||15||11.3||116.88||2||0.71||70+|
Dams in our study reaches are predominately small dams that have relatively little storage capacity and hence little influence on flow distribution or flow frequency. For example, six dams in our study are less than 2 m high, two dams are between 2 and 3 m high, three dams are between 3 and 4 m high, and the remaining four dams are higher than 4 m (Table 1). The hydraulic residence times for the impoundments of nine of the 15 sites are two days or less. Most of the dams release water from spillway overflows. The ages of the dams varied: four of the dams are more than 100-years old, seven of the dams are between 50-years and 100-years old, and three of the dams are between 10-years and 50-years old. The age of one dam could not be determined.