Are national barrier inventories fit for stream connectivity restoration needs? A test of two catchments

Abstract

Catchment-scale river reconnection programmes require barrier inventories for restoration planning, yet barrier inventories are variable in extent and quality internationally. To test the degree to which barrier databases, in this case for England, are fit for purpose, we made a comparison of the national database (mostly originating from desk-study) for two catchments, the Wear and the Tees, against detailed walkover surveys. We surveyed 701 km (32.8%) of stream length, stratified by stream order, altitude and subcatchment and recorded natural and artificial barriers. Only 22.7% of barriers identified in the walkover survey were present in the national database, including low-head (<5 m) artificial structures (32.3% representation), artificial barriers ≥5 m (14.3% representation) and culverts (0% representation). About 18.9% of artificial barriers in the national database were found, during a field survey, to have been breached naturally. Mean densities of artificial barriers were 0.68 barriers km −1 and 0.45 barriers km −1 in the Wear and Tees, respectively, significantly higher than in the national database. Stream connectivity restoration in England may be hampered by the incomplete national barrier inventory; we recommend careful checks of barrier inventories as they are developed internationally.

Broken rivers: ground-truthing the world’s most fragmented rivers

“…We identified 629,955 unique artificial barriers in 36 countries (Figure 1 ), after excluding 106,393 duplicates (see Methods). This figure is one order of magnitude higher than previous estimates of longitudinal fragmentation for Europe based only on large dams 15,16 , but consistent with regional 33,35,36 and country estimates that considered all barriers 26 . We combined over 1,000 different barrier types into six main functional categories that capture variation in barrier use and size 26 (Table 1 ).…”

Rapid response of fish and aquatic habitat to removal of a tidal barrier

“…On the River Tees, only two out of 20 barriers where connectivity has been restored have been removed, with the remainder installed with fish passes (Sun, Galib & Lucas, 2020). This proportion is probably typical of European and North American rivers.…”

“…That risk applies much less to small barriers, which do not retain large amounts of fine‐sediment deposits behind them. The vast majority of artificial river barriers are small (Januchowski‐Hartley et al, 2013; Jones et al, 2019; Sun, Galib & Lucas, 2020). Nevertheless, although removal of redundant barriers is a preferred restoration tool, in many cases barriers cannot be removed owing to societal needs or because of constraints such as erosion risks on nearby infrastructure (Birnie‐Gauvin et al, 2017b).…”

“…Nevertheless, although removal of redundant barriers is a preferred restoration tool, in many cases barriers cannot be removed owing to societal needs or because of constraints such as erosion risks on nearby infrastructure (Birnie‐Gauvin et al, 2017b). On the River Tees, only two out of 20 barriers where connectivity has been restored have been removed, with the remainder installed with fish passes (Sun, Galib & Lucas, 2020). This proportion is probably typical of European and North American rivers.…”

“…Instream obstacles such as dams, weirs, and culverts fragment rivers by interrupting longitudinal connectivity and altering habitat (Nilsson et al, 2005;Sun, Galib & Lucas, 2020), having major effects on the biodiversity and functioning of river ecosystems (Bunn & Arthington, 2002;Pringle, 2003;Galib et al, 2018). These obstacles frequently affect the dispersal and migration of fish species, and can result in population decline and biodiversity reduction (Lucas & Baras, 2001;Gehrke, Gilligan & Barwick, 2002;Katano et al, 2006;Mueller, Pander & Geist, 2011).…”