Impediment and Habitat Inventories

Fish Passage Impediment Classification and Prioritization

In general, fish passage impediments are classified as either natural impediments (e.g. natural waterfalls, extremely high gradient stream reaches), small scale impediments (e.g. log and/or debris jams, landowner-constructed stream crossings, reaches of invasive vegetation), or large scale impediments (e.g. low flow dams, improperly placed or sized road and stream crossing culverts or bridges, snowmobile trail crossings, improperly placed stone ford crossings, berms, dikes, levees). The Program typically does not suggest any changes to allow fish migration around natural fish passage impediments. Small scale impediments are those that can generally be removed or remediated largely with hand labor and hand tools. In general, large scale impediment removals or remediation are addressed with heavy equipment due to the scope and scale of the projects. In general, impediment removal or remediation projects are prioritized based on a number of factors including a Department-developed Stream Reach Prioritization Methodology, landowner cooperation, site conditions and accessibility, cost, available funding, etc.  Connectivity between the AOC and Lake Michigan and identified potential northern pike spawning habitats, as well as potential restorable wetland habitat qualities, is also considered. Impediment removal prioritization also considers the lateral hydrologic connectivity (i.e., the level of channel entrenchment and duration and magnitude of surface hydrology connection between floodplains and channels) of habitat areas to their respective stream channels, the existing quality or restorative potential of riparian wetlands and other suitable northern pike spawning habitats, and the documented occurrences of fish and wildlife species indicative of desirable habitat qualities. In general, full removal of an impediment and restoration of the stream and riparian areas is preferred.


Habitat needs to be accessible for all critical life stages to be ecologically valuable to a sustainable fishery. For example, a fish spawning area may be accessible to adults during typical high-water spring flows and provide ideal habitat for juvenile fish to develop, but may become isolated during low water periods preventing young-of-the year to migrate downstream during a critical period. Therefore, ingress and egress to critical habitat are equally important. Various fish species have very different behaviors and physical abilities pass obstacles. Northern pike (Esox lucius) is a native potamodromous fish that inhabits nearshore waters of Lake Michigan, the Milwaukee River estuary, and is also a year-round resident of the river itself. These fish commonly enter very small, oftentimes intermittent, streams to access wetlands and seasonally flooded areas where they spawn. Even though this fish can swim very fast, it can do so only for short distances. In fact, northern pike are one of the weakest swimming native fish in passing long stretches of higher velocity water. In addition, it will not leap to pass cascading water features. For these reasons, northern pike are used as a surrogate to evaluate impediments for all species, and northern pike swimming abilities are considered in all fish passage removal or remediation designs.

The Ozaukee County Planning and Parks Department routinely conducts aquatic habitat and fish passage impediment inventories, which are completed both remotely (e.g. orthophoto analysis) and “on the ground” in the field. In general, potential northern pike spawning areas are identified by qualitative assessments of: 1. Lateral channel connectivity to riparian areas (e.g. floodplain and wetlands) during regular high flows (i.e., channel entrenchment); 2. Herbaceous hydrophytic vegetation density in the channel, on banks or floodplain, or in a littoral area; and 3. Regular riparian flooding (e.g., evidenced by watermarks on trees, drainage patterns, etc.).

Tier I and Tier II Habitat and Inventory Assessment

Similar to habitat inventories, field inspections are used to confirm the presence of suspected impediments. Impediment inventories in the field typically include a rapid, “Tier 1” inspection to determine if a suspected impediment (identified remotely) exists. A more detailed “Tier II” assessment, which was developed in conjunction with partners including the US Fish and Wildlife Service, is an ecosystem-based approach to collected detailed impediment data at road and stream crossings to facilitate “no-slope” and “stream simulation” designs for impediment removal or remediation.

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A summary of these protocols is provided below:

Inspecting aerial photographs yields a list of potential sites for each project stream. Rapid, Tier 1 field inspections and/or local resident interviews are used to determine if a suspected impediment exists at each potential site, as well as to confirm areas that either meet the general criteria of northern pike spawning habitat or are recently/historically observed to host spawning northern pike. 

A standard field form and guidance document for Tier 1 field inspections is used. Using the County GIS and tax assessment database, Program staff contacts all owners of riparian land immediately adjacent to suspected impediments prior to visiting the site to gather additional information about the suspected impediment and to gain access permission. All non-inspected potential sites that cannot be confirmed or refuted by direct onsite inspection or with information from resident interviews are considered suspected impediments until evidence to the contrary becomes available.

All suspected impediments are thoroughly photographed. Latitude and longitude coordinates are determined as needed. If possible, impediments are marked with survey flagging for easier relocation. Applicable information is collected and recorded on the Tier 1 assessment field sheet standardized field data sheets. In certain cases (e.g., low flow dams and impassable culverts) staff returns with survey equipment and completes a single Tier 2 assessment, which provides critical quantitative site information about each impediment that can be used to support the engineering and design process.

Forms Used for Stream Inventory

As of September 2020, Planning and Parks Department staff has inventoried approximately 678 suspected impediments on 42 streams in Ozaukee County in the direct Lake Michigan drainage basin and the Milwaukee River watershed. The inventory information is saved and organized in a Planning and Parks Department GIS database.

Inspecting Aerial Photos

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Completing Tier 1 Form

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Program Staff Assessing Culverts

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