Aquatic Fragmentation

Framework / Assessment
California Water Plan Update 2013: Pilot Statewide Assessment
Summary

Aquatic fragmentation in a watershed or hydrologic region. When streams are crossed by roads or dams, the portions above and below the potential barrier are separated from each other in a process called fragmentation. This can interfere with physical processes and movement of aquatic organisms.

General Information
Sustainability Goal

Goal 5. Protect and enhance environmental conditions by improving watershed, floodplain, and aquatic condition and processes.

Domain

Ecosystem Health = The condition of natural system, including terrestrial systems interacting with aquatic systems through runoff pathways.

What is it?

Aquatic fragmentation is the potential hydrologic alteration caused by diverse type of structures, such as dams, weirs, drop structures, and other man-made systems that modify hydrologic flow. It is an influence indicator that is directly or indirectly connected to effects on aquatic habitat functioning and species condition. It also represents the impact of development and/or land use in the watershed. The effects of structures are not limited to roads. Other disturbance features, such as seismic lines, pipelines, and rail lines, have been shown to have both direct (increased mortality) and indirect (avoidance of high quality habitat) effects. The aquatic fragmentation indicator identifies the proportion of the watershed or stream segments unfragmented by dams and road crossings of streams. A complementary metric is the density of road/stream intersections within a watershed area.

Why is it important?

Streams and rivers may be disconnected by physical and other barriers. Dams, culverts, in‐stream impoundments, high temperature, and excessive aquatic plant growth can all separate waterways into segments (Bourne et al 2011). Fragmentation caused by these natural or artificial barriers cause different effects in watershed health and wildlife that depend on it.

Changes in physical, geomorphological and chemical properties of watersheds are one type of aquatic fragmentation impacts. Natural processes are also altered by the physical and structural changes in watershed and consequently, aquatic organisms and their life cycles are also impacted. Locations where roads cross waterways change the natural shape of the river and how it is allowed to flow through the barrier. This can increase sediment transport and deposition and erosion in riparian habitats (Warren and Pardew 1998, Forman and Alexander 1998). Increases in sedimentation lead to changes in flow regime and water stability, stream channel instability, and reduced water quality (Rieman and McIntyre 1993). An increase in fine sediments, particularly in small spawning streams, can have negative impacts on fish egg survival and spawning success and may directly kill aquatic organisms (Newcombe and Jensen 1996).

Aquatic fragmentation has direct and indirect effects on the ecology, diversity and abundance of a variety of aquatic organisms. Andrew and Wulder (2011), for instance, analyzed the relationships between the population trends of Pacific salmon, from 1953 to 2006, and land cover, fragmentation, and forest age. Their results showed that effects are species specific, but characteristics indicating a legacy of historic and current forest management generally had negative effects, driven by a small subset of highly fragmented watersheds. In particular, the results showed that chum and coho salmon had strong negative relationships with fragmentation. Bain and Wine (2010) studied watershed in the Hudson River and found out that large stream fragments support higher species diversity, more abundant populations, and a greater range of fish sizes.

In addition, the movement and migration of aquatic species is altered due to aquatic fragmentation. Crossings and higher barrier frequency could be associated with increases in the water velocity due to the configuration of a road crossing and are inversely proportional to fish movement (Warren and Pardew 1998). Raymond (1979) and Fergusson et al (2006) have documented that turbines and dams have adverse effects on survival and migration of juvenile salmon, mainly chinook and steelhead, in the Columbia River system.

Roads can also increase the risk of overharvesting for many game fish species (i.e. lake trout and bull trout); for example, road densities as low as 0.1km/km2 have been found to negatively influence trout populations, and new road access into previously remote aquatic habitats can increase angling and poaching mortalities (BCMWLAP 2002) .

In summary, whole watershed connectivity is critical for effective conservation of rivers and networks of wetlands to ensure natural processes (Moilanen et al. 2009; Nel et al. 2009); including upstream connectivity, maintenance of biological diversity, fish migratory routes, free-flowing rivers, significant water yield areas and water quality.

What can Influence or Stress Condition?

The desired condition of an unfragmented watershed system can be influenced by any type of structure or barrier that disconnect or limit the natural flow of the waterway and will affect directly or indirectly its biological and physical features. Large and small barriers should be considered when evaluating riparian conservation efforts considering that both types of structures have effects on wildlife (Tiemann et al 2004) in the watershed.

Basis of Calculation and Use

The proposed scoring system for aquatic fragmentation comes from two distinct methods. The first involves a percentage of the HUC 12 (2012) watershed that is “unfragmented”, that is, above a disturbance site. In this analysis, the Passage Assessment Data (PAD) (2013) is used to demarcate new watersheds, referred to here as “PAD watersheds”. All watersheds created by the PAD data points represent areas of the HUC 12 that are separated from the rest of the HUC 12 watershed downstream. In some cases these PAD watersheds are much smaller than the HUC 12 watersheds, in others they are much larger. An additional measure is also used, the density of road/ stream intersections within each HUC 12 watershed in a standardized per unit length of stream as determined by the National Hydrography Dataset (NHD) (2013), or density per unit watershed area. These two methods are combined to create a scoring system by which each HUC 12 watershed within the area of interest is ranked.

Target or Desired Condition

The desired condition, from an ecological health standpoint, is that waterways in local, regional and statewide scales have a minimum or no fragmentation, so they can conserve or resemble the historical natural watershed connectivity that will allow aquatic species and systems to function correctly. The target condition is that 100% of the watershed is unfragmented and the density of road/stream intersections and dams is 0 crossings/km2, representing a score of 100. The corresponding undesired condition or target is a density of fragmenting elements that blocks natural movement of aquatic organisms. After review of the literature on road-stream crossings, Fiera (2012) used a value of 0.6 crossings/km2 to represent a “high pressure” on aquatic biodiversity; which is the value used here to represent a score of 0. A qualification on this approach is that if roads intersect streams via a bridge or causeway that spans the floodplain, then the fragmenting effect of the road may be minimal or nil. Therefore, a modification of the desired condition target is that all road-stream intersections are composed of crossing structures that either span the floodplain or demonstrably do not inhibit functional connectivity of upstream and downstream areas.