Barriers to Dispersal. Dispersal of mussels is dependent on movement of host fish and serves several important functions such as connecting subpopulations within the occupied range of a species or allowing a species to move into formerly uninhabited areas (Xxxxxxx, 2008). Degradation and loss of habitat due to anthropogenic actions may lead to large sections of unsuitable mussel habitat, thus reducing dispersal success (Xxxxxxx, 2008). Dams can act as permanent barriers to host fish movement, and hydroelectric dams may impinge or entrain hosts and result in mortality (Xxxxxxx, 1996; Xxxxxx et al., 2008; Xxxxxxxxx et al., 2017). The underlying geology of the Middle Basin may affect host fish migration under certain circumstances, such as during low and base flow conditions. A portion of the Middle Trinity has marl outcrops which create small waterfalls and very shallow riffles during base and/or low flow conditions. Natural physical barriers, such as waterfalls associated with resistant geological formations, are thought to potentially restrict host fish movement and subsequently affect the structure of mussel communities although recent genetic diversity studies have produced conflicting evidence regarding gene flow among certain species of host fish and unionids (Xxxxx and Xxxxxx, 2005; Xxxxxxxx et al., 2007; Xxxxxxxxx et al., 2012). Evidence presented by Xxxxxxx (1996) suggested that low head dams as small as 1 meter high are problematic for the distribution of host fish, particularly for benthic species, such as freshwater drum. Given the apparent similarity in structure and hydrological function, the geological outcrops along the Middle Trinity could present similar migratory obstacles. Also located in this reach are four river locks (Locks 2 through 5), which likely were constructed on similar geological formations. The existing locks, although continuously left open since construction in the early 1900s, may be contributing similarly in part as a migratory barrier. Low head dams are also in the Upper Basin for grade control in the Fort Worth Floodway and as part of existing and historical water supply intake structures.
