River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
3. Rivers and streams in the Wellington region
This section presents a brief overview of the natural diversity of river and
stream types/environments in the Wellington region, providing context for
some of the natural differences in water quality and ecological health observed
across the region’s rivers and streams. Although Greater Wellington’s RSoE
network focuses on permanently flowing water bodies, both permanently and
intermittently flowing streams are outlined. This is consistent with the RMA
(1991) definition of a river or stream as “a continually or intermittently flowing
body of fresh water” that includes streams and modified watercourses. Some of
the key anthropogenic activities that can affect stream water quality and
ecological health are also outlined, with a particular focus on land use and
significant consented activities in the Wellington region.
3.1 Permanently flowing rivers and streams
There are approximately 12,360 km of permanently flowing rivers and streams
in the Wellington region. These rivers and streams range from the small-sized
first order streams, found in headwaters and springheads, through to the larger
lower reaches of major rivers such as the Otaki, Hutt and Pahaoa rivers (sixth
order) and the Ruamahanga River (seventh order). First and second order
streams make up approximately 75% of the entire length of rivers and streams
in the region while rivers belonging to orders 5, 6 and 7 make up only 5% of
total river and stream length (Table 3.1).
Table 3.1: Approximate lengths of rivers and streams belonging to each order in
the Wellington region. Data obtained from REC (Snelder et al. 2004)
Order Approximate length (km) River/stream length in the region (%)
1 6,037 49
2 3,130 25
3 1,575 13
4 928 8
5 427 3
6 174 1
7 89 1
Total 12,360 100
Differences in geophysical characteristics of river and stream catchments, such
as climate, topography and geology, result in natural differences in river and
stream characteristics (Snelder at al. 2004), including differences in flow,
slope, temperature and substrate. This, together with downstream factors, such
as distance from the coast, affect the composition of biological communities
including instream plants, macroinvertebrates and fish (Leathwick et al. 2008).
Rivers and streams in the Wellington region can be broadly grouped into three
types based on their catchment geophysical characteristics:
Steep gradients and hard sedimentary geology (Figure 3.1a and 3.1b) – these
rivers and streams are found in the Tararua, Aorangi and Rimutaka ranges as
well as in moderately elevated areas around the Wellington coast and eastern
Wairarapa. They have naturally high water clarity and low concentrations of
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