Influence of regional geology and hydrogeology on river habitat quality: examples from Mill Brook and Elderkin Brook, Kings County, Nova Scotia

Elderkin Brook and Mill Brook are tributaries of the Cornwallis River and w ere investigated with the intent of understanding the geological conditions that could influence both habitat quality and river restoration efforts. T he Cornwallis River and its tributaries have been the focus of restoration efforts in an attempt to re-establish brook trout (Salvelinus fontinalis) a nd Atlantic salmon (Salmo solar) populations. Both tributaries are relative ly short, high-gradient headwater streams that are ungraded along much of t heir length. Data from Elderkin Brook indicate that pyrite-rich slate of the Kentville a nd Halifax formations that dominate the watershed geology do not govern the pH of the river discharge. The carbonaceous New Canaan Formation buffers r egional groundwater and has resulted in elevated pH levels (7.5-8.0). Sprin gs in the ungraded portion of Elderkin Brook introduce slightly acidic iron -rich groundwater into the system and, upon contact with the alkaline disch arge, have produced sporadic masses of flocculated ferric hydroxide (Fe(OH) (3)) Along the upper reaches of Mill Brook, iron-rich acidic groundwater (p H 5.0-5.5) originating from the Kentville Formation influences the chemistr y of the river discharge during low-flow conditions. However, groundwater i nput from the porous, calcareous-cemented Wolfville Formation that underlie s the lower portion of Mill Brook has resulted in locally elevated pH level s (6.0-6.5) and the precipitation of extensive Fe(OH)(3) deposits. In both Elderkin Brook and Mill Brook, the precipitation, deposition and pr eservation of ferric hydroxide is controlled by regional and local, geologi cal and hydrogeological conditions. During the winter and spring, increased discharge and dilution of the groundwater chemistry by precipitation keep iron in either solution or suspension. During low-flow periods (primarily i n the fall) flocculated ferric hydroxide accumulations develop coincident w ith the spawning of both fish species. These conditions result in both temp orally and spatially constrained habitat degradation that may be difficult to recognise using the habitat evaluation techniques that are presently bei ng employed by river management groups.