There are many frameworks that may be used to categorise estuaries. Usually, the categories reflect differences in the geological morphology of the estuary (i.e., its broad physical features such as shape and depth).
Understanding estuary types can be helpful in interpreting monitoring results, as some types of estuaries are more prone than others to the accumulation of mud, chemical contaminants and nutrients.
Estuary Trophic Index
The broadest classification system commonly used in Aotearoa New Zealand is described in the Estuary Trophic Index (ETI) tool. The ETI defines four categories of estuarine systems which have similar physical features, namely depth, extent of intertidal areas, and flushing time. This classification system was designed primarily to help assess the risk of eutrophication. The four categories are described in Table 1.
Table 1. Estuarine categories defined in the Estuary Trophic Index (Tool 1).
|Shallow intertidal dominated estuaries (SIDEs)||
Shallow, short residence time (<3 days), and predominantly intertidal tidal lagoons.Examples: Freshwater Estuary (Stewart Island), Manukau Harbour (Auckland), Havelock Estuary (Marlborough), Kaikorai Estuary (Otago).
|Deeper subtidal dominated, longer residence time estuaries (DSDEs)||
Mainly subtidal, moderately deep (>3m to 15m mean depth) coastal embayments and tidal lagoon estuaries, with moderate residence times >7 to 60 days).Examples: Firth of Thames (Waikato), Akaroa Harbour (Canterbury), Wellington Harbour (Wellington).
|Shallow, short residence time tidal river with adjoining lagoon estuaries (SSRTREs)||
Shallow, short residence time (<3 days) tidal river estuaries (including those that exit via a very well-flushed small lagoon). Tidal River ICOLLs with closure periods of days rather than months and high freshwater inflows can also fit in this category.
|Intermittently closed/open lakes and lagoons (ICOLLs)||
Shallow tidal lagoon (also called coastal lakes) and tidal river type estuaries (<3m deep) that experience periodical mouth closure or constriction. In general, the tidal river ICOLLs have shorter periods of mouth closure (unless they are very small) than the tidal lagoon ICOLLs.Examples: Waituna Lagoon (Southland), Lake Onoke (Wellington).
The short residence times of SIDEs and SSRTREs mean the water in the estuary is fully replaced regularly. This makes the risk of pollutants building up in the estuary lower than in DSDEs, for instance, where longer residence times give pollutants more time to settle out onto the seafloor or, in the case of nutrients, contribute to nuisance blooms of algae. SIDEs are the most common type of estuaries in Aotearoa New Zealand.
Coastal hydrosystem classification
A more detailed framework than that described in the ETI is the ‘coastal hydrosystem classification’, published in 2016 by the Ministry for the Environment. This classification scheme is based on a hierarchy of physical features and categorises all coastal hydrosystems (going beyond estuaries) into one of 11 classes. The aim is to provide a consistent terminology for referring to coastal waterbodies. It is also intended that by providing a means of identifying similar systems that may be managed in a similar way, the scheme will enable coastal management to be more strategic. Brief descriptions of the classes are given in Table 2 and more information, including about the different subclasses within these, can be found via the “Read more” section.
Table 2. Geomorphic classes described in MfE’s coastal hydrosystems classification scheme.
|Damp sand plain lake||
Also known as dune lakes or coastal lakes, these are small, shallow (1−2m deep), typically freshwater bodies. They have no connection to the sea and are often found in depressions between sand dunes. As such, these systems are not estuaries.Examples: Lakes on Kaipara’s North Head (Northland) and Farewell Spit (Golden Bay)
These large (several km2), shallow (2−3m deep) coastal lagoons are barred from the sea by a barrier or barrier beach. The waterbody is typically fresh and fed by small streams, with occasional brackish pockets. These systems are not estuaries.Examples: Te Waihora Lake Ellesmere (Canterbury), Ohuia Lagoon (Hawke’s Bay), Waituna Lagoon (Southland)
Narrow (10s to 100s m), elongated (<100m to several km long), and shallow river mouth lagoons that are formed by strong longshore sediment transport. There is usually no tidal inflow, therefore these systems are not estuaries.Examples: Waitaki and Ashburton Rivers (Canterbury)
Very shallow streams that flow over the beach face (as opposed to cutting a subtidal channel through the sand or gravel to the sea). These systems are not estuaries as there is no tidal inflow, except during storm events that are coupled with high tides.Examples: Stony River (Taranaki), Piha Stream (Auckland), Mohikinui River (West Coast)
|Freshwater river mouth||
Rivers where flow is large enough to cut a permanent subtidal channel across the shoreline and to the sea. There is a permanent connection to the sea, and the river channel is steep enough to prevent tidal inflow. These systems are not estuaries.Examples: Clarence River (Canterbury), Tapu Stream (Waikato), Haast River (West Coast)
|Tidal river mouth||
Elongated, narrow and shallow basins that have a permanent connection to the sea. River flow is a significant proportion of the basin’s volume and is greater than the tidal volume entering, yet these systems are estuaries.
Examples: Hutt Estuary (Wellington), Manawatū Estuary (Manawatū-Whanganui), Whakatane Estuary (Bay of Plenty)
Shallow (1−3m deep), circular to elongated basins with simple shorelines and extensive intertidal area. The entrance to the sea is narrow, constricted by a spit or sand barrier. Strong reversing tidal currents flow through the entrance, and river input is small compared to the tidal inflow. These systems are estuaries.
Examples: Blueskin Bay (Otago), Whangateau Harbour (Auckland), Tairua Harbour (Waikato), Te Awarua-o-Porirua Harbour (Wellington)
|Shallow drowned valley||
Shallow (<5 m deep) with complex shorelines and numerous narrow arms leading off a main basin or channel. Extensive intertidal flats that are cut by drainage channels. Can vary in size from small tidal creeks to large harbours. These systems are estuaries.
Examples: Kaipara Harbour (Northland/Auckland), Mangemangeroa Creek (Auckland), Raglan Harbour (Waikato)
|Deep drowned valley||
Large, deep (10−30m deep), mostly subtidal systems. Both river and tidal inputs are small proportions of the total basin volume. These systems are estuaries.
Examples: Firth of Thames (Waikato), Wellington Harbour (Wellington), Akaroa Harbour (Canterbury)
Long, narrow and very deep (70−140m) U-shaped basins with steep sides or cliffs. The basin is subtidal with only small intertidal areas in the headwaters. Both river and tidal inputs are small proportions of the total basin volume. These systems are estuaries.
Examples: Milford Sound (Southland)
An indentation in the shoreline with a wide entrance, bounded by rocky headlands and open to the ocean. The waterbody is shallow to medium depth and circular or elongate. There is little river influence, making these systems marine rather than estuarine.
Examples: Matai Bay (Northland), Lyall Bay (Wellington), Rocky Bay (Waiheke Island).
Considering scale can be important when assessing estuary types, as very large estuaries may contain subsystems or areas that represent different classes. For example, the Kaipara and Manukau Harbours are both shallow, drowned valleys at the whole-estuary scale, yet within the Kaipara Harbour there are areas that would be classed as tidal river mouths (e.g., Kaipara River), tidal river lagoons (e.g., Waionui Inlet) and small shallow drowned valleys (e.g., Tauhoa River). Likewise, the Manukau Harbour contains tidal rivers (e.g., Huia Stream) and small shallow drowned valleys (e.g., Pāhurehure Inlet).
The most appropriate way to classify an estuary depends on the application. If estuary-wide management is being considered, then whole-estuary classifications will be most suitable to understand how interventions might play out. However if the interest is in more local management, for example a habitat restoration project in the Waionui Inlet of the Kaipara Harbour, it may be more helpful to use the inlet-scale classification of tidal river lagoon to understand the best approach.
Robertson BM, Stevens L, Robertson B, Zeldis J, Green M, Madarasz-Smith A, Plew D, Storey R, Hume T, Oliver M 2016. Estuary Trophic Index Screening Tool 1. Determining eutrophication susceptibility using physical and nutrient load data. Envirolink Tools Project: Estuarine Trophic Index, MBIE/NIWA Contract No: C01X1420.