Introduction
Dams cause physical and ecological impacts to aquatic ecosystems
disrupting over half of the world’s major river systems (Nilsson et al.,
2005). Although dams vary in their characteristics (e.g., size,
function), they have some common effects on habitat, discharge, and
energy inputs. All dams interrupt the movement of water and aquatic
organisms and can disrupt energy and nutrient flows. Organisms in river
systems rely on energy inputs from both upstream and downstream
habitats, and structures interrupting river connectivity generally have
watershed-scale effects (Vannote et al., 1980; Ward, 1989; Stanford &
Ward, 2001; Schindler et al., 2013). Upstream, inundation eliminates
terrestrial habitats, increases water depth, and removes turbulent flow,
significantly affecting lotic specialists (Nilsson et al., 2005).
Downstream, releases from the dams alter habitats, create unnatural
temperature variation, alter natural flow variation, limit channel
development/maintenance, and reduce system productivity (Nilsson et al.,
2005). The serial discontinuity concept (SDC) details theoretical
ecosystem responses to altered conditions downstream of such
disturbances, identifying how certain variables (ecological
connectivity, biodiversity, particulate matter abundance) are expected
to decrease in tailrace areas before gradually increasing along a
longitudinal gradient (Ward & Stanford, 1989; Stanford & Ward, 2001).
Hydropeaking dams in particular can have effects on downstream waters,
including artificial flow variation and alteration to the natural
thermal regime (Irwin & Freeman, 2002; Twardek et al., 2022). R.L.
Harris Dam on the Tallapoosa River in east-central Alabama is such a
facility that operates on a hydropeaking schedule. Constructed in 1983,
the dam initially operated with hypolimnetic release and no partial flow
between generation periods which resulted in both thermal and flow
modification of the downstream area. In 2005, stakeholders adopted an
adaptive management plan, (called the “Green Plan”) that established
regulations on dam operations, including base flow requirements between
generation periods (Irwin & Freeman, 2002; Kleinschmidt Associates,
2018). Although the Green Plan mitigated some of the effects of dam
operation, water was still released from the hypolimnion, leading to
pulses of higher flow with colder water temperatures during summer.
Downstream effects of dam operations under such a modified hydropeaking
schedule on fishes have not been thoroughly described prior to this
work. The subsequent change in downstream water temperature between
generation versus non-generation periods could reach 10°C and the change
remained measurable many kilometers downstream (Irwin & Freeman, 2002),
although the majority of hourly temperature changes were less than 2°C
(DeVries et al. 2022).
Alabama is a hotspot of aquatic biodiversity, and each river system in
the state has unique aspects of its native fish assemblages (Lydeard &
Mayden, 1995; Mettee et al., 1996; Freeman et al., 2005). Historically
as many as 126 fish species have been identified in the Tallapoosa
River, and the broader Mobile drainage is one of the most diverse
freshwater fish assemblages in North America (Lydeard & Mayden, 1995;
Travnichek & Maceina, 1994; Freeman et al., 2005). Protecting
biodiversity in Alabama rivers is increasingly important as
anthropogenic impacts increase. To protect aquatic resources, it is
necessary to identify how various threats (such as a dam) affect the
ecology and life history of resident organisms. The Tallapoosa River
from the tailrace of Harris Dam downstream to the headwaters of Lake
Martin is diverse in both habitat and resident fishes and is considered
one of the highest quality river segments in the piedmont region of
Alabama (Irwin & Freeman, 2002). Although little information about the
Tallapoosa River fish assemblage is available from before Harris Dam was
constructed, several studies of the fish assemblage have been conducted
since 1983 (Kinsolving & Bain, 1993; Travnichek & Maceina, 1994; Irwin
& Freeman, 2002; Freeman et al., 2005; Irwin et al., 2019). Beyond the
effects of hydropeaking dam operation discussed earlier, Travnichek and
Maceina (1994) showed that the diversity and richness of fish inhabiting
shallow downstream waters was decreased in regulated portions of the
Tallapoosa River versus upstream/unregulated areas. However, little work
has been conducted after the implementation of the modified hydropeaking
operations on the overall fish community; as such, we quantified the
fish assemblage across a spatial gradient downstream from the dam under
the current “Green Plan” to determine the magnitude of continued
effects of Harris Dam.