Small dams need consideration in riverscape conservation assessments

Abstract

1. Small, off-channel dams are generally ignored in impact assessments owing to limited information and spatial resolution issues. Previous research on South African rivers showed correlative links between high density of small dams and associated reductions in low flows, poorer water quality, and impoverished aquatic macroinvertebrate communities that were dominated by opportunistic taxa instead of specialist groups.2. Since small dams are usually associated with catchment transformation (for example, vineyards, stock farming and exotic timber plantations), they are convenient surrogates of the impacts of catchment transformation on river functionality. Here, an index of cumulative small dams for South Africa, Lesotho and Swaziland is presented and evaluated.3. Fifty-two per cent of the water management catchments in the study region exceeded the threshold for the cumulative small dams density (SDD) index above which river functionality is compromised. This estimate of potentially affected catchments is considered to be conservative for reasons discussed. 4. The index results are compared with a recent systematic biodiversity planning exercise for setting biodiversity targets for freshwater areas of South Africa. Although the systematic planning included in-stream small dams within 50 m of a river, analysis showed that 36% of all quaternaries that have high SDD score overlap with river reaches classified as 'natural' or 'largely natural'. 5. Disregarding dams outside the 50 m buffer area equates to ignoring the majority of small dams (94%) in South Africa, and it is recommended that aquatic conservation assessments include the SDD index as a cost layer for prioritizing rivers for rehabilitation and conservation.

Freshwater conservation in a fragmented world: Dealing with barriers in a systematic planning framework

“…The adequate consideration of barriers in conservation planning will need good inventories of their spatial location, permeability characteristics and potential impact on biodiversity (Januchowski‐Hartley et al, ; Januchowski‐Hartley et al, ; Mantel, Rivers‐Moore, & Ramulifho, ). Here, the spatial location of large dams in the Iberian Peninsula was used for demonstration purposes and for consistency in data quality across the whole study area.…”

Regulation of carbon dioxide and methane in small agricultural reservoirs: optimizing potential for greenhouse gas uptake

“…The global abundance of these systems remains uncertain (Verpoorter et al, 2014), but statistical extrapolation suggest there may be around 16 million artificial reservoirs worldwide (Lehner et al, 2011). Regional-scale inventories indicate that collectively upwards of 8 million farm reservoirs exist in the USA (Brunson, 1999;Smith et al, 2002), China (Chen et al, 2019), India (Anbumozhi et al, 2001), South Africa (Mantel et al, 2017), and Australia alone (Lowe et al, 2005;MDBA, 2008;Grinham et al, 2018a). The density of farm reservoirs can exceed 30 % of agricultural area in some regions such as China where food demand is high (Chen et al, 2019).…”

Modelling built infrastructure heights to evaluate common assumptions in aquatic conservation

“…Scientists and practitioners require information on the characteristics of built infrastructure, such as dams and weirs, to better understand associated impacts, costs, and benefits, in relation to ecological processes, services, and human values (Poff & Hart, 2002;Januchowski-Hartley et al, 2013;Major et al, 2017). Characteristics of larger built infrastructure are increasingly well understood, because of improved identification via remotely sensed imagery (Mantel et al, 2017), and superior record keeping due to the importance of size and water holding capacity for monitoring energy production and water storage (e.g., Carvajal et al, 2017). Despite likely impacts from small built infrastructure which often restrict fish movement (i.e., being impassable), impede river flows, and retain sediments and materials, data on their distribution and characteristics remain limited (Januchowski-Hartley et al, 2013;Couto & Olden, 2018).…”