The natural vegetation of the Denmark River catchment is predominantly forest with a great range of forms and diversity of species. Throughout the area there are also broad swampy drainage lines which carry paperbark or banksia woodlands. The majority of the River flows through the Mount Lindsay National Park.

Although only a 28% of the upper catchment has been cleared, it has been sufficient to affect the quality of water collected in dam built to supply the township of Denmark. The River's salinity has increased from about 40 mg/litre in 1940 to about 500 mg/litre in the early 1990s. For this reason catchment clearing control has been imposed and commercial tree planting carried out in the upper catchment to prevent further deterioration and to improve water quality.

The Quickup River is a tributary to the Denmark River located four kilometres north of Denmark. A dam located in lower section of the catchment is the main water supply for Denmark.

The Denmark River flows all year round and is one of the two major tributaries to the Wilson Inlet.

The River was sampled in November 2007.

Western Minnow and fresh water crayfish species Cherax crassimanus were found at all sites. The Western Pygmy Perch and Night Fish were found at both DEN01a and QUI01 and Balston's Pygmy Perch was also recorded at DEN01a. Fresh water crayfish species Cherax preissii was recorded at DEN08 and QUI02. Pictures of these can be viewed in the gallery.

The four sites sampled were all determined to be in pristine condition and were graded in a A-grade condition.

All sites on the Denmark River were recorded brackish levels (6.5 - 8.2mS/cm) of conductivity and all sites on the Quickup River recorded fresh water levels (0.7 - 1.0mS/cm).

All sites sampled were acidic with a pH ranging between 4.16 and 6.5.

DEN01a and QUI01 recorded high levels of turbidity and DEN08 and QUI02 recorded low levels.

All sites apart from DEN01a recorded low levels of total phosphorus and all sites apart from DEN08 recorded low levels of total nitrogen.