Stormwater ecosystems

Hobart's rivulets are steep, rocky, freshwater ecosystems. They have pools and riffles, where fast flowing water moves over and around shallower, rocky areas.

The steep terrain has resulted in many small waterfalls. The shape of the landscape gives each rivulet just a short run before it enters the timtumili minanya/River Derwent.

Forest, including wet temperate rainforest, provides plenty of shading for the rivulets. The forest results in plenty of leaf litter and woody debris. This creates an abundance of different habitats for the organisms that live in Hobart's rivulets.

Typically, leaf litter and sticks washed down from surrounding forest would provide food and habitat for aquatic life in Hobart's rivulets. There would be relatively few plants and algae living within the rivulet system due to low light levels and the relatively fast flowing nature of these waterways.

A wide variety of animals make their home in Hobart's rivulets, including:

• platypus
• rakali (water rats)
• several species of jollytail (Galaxias spp.)
• wide diversity of waterbugs (also known as freshwater macroinvertebrates).

In an urban waterway, everything from habitat to water quality tends to be modified.

In areas with lots of infrastructure, waterways tend to be managed. The aim is to minimise the risk of flood damage on nearby structures, including roads, houses, buildings. These modifications change and limit the habitats available in a waterway. This usually reduces the diversity of aquatic wildlife, including native fish and waterbugs.

Urban waterways are characterised by much faster flowing water systems.

Concrete and tarmac shed rainwater swiftly. Stormwater systems are designed to remove standing water as quickly as possible.

Faster moving water often makes urban rivulets highly erosive places. Channels are forced to move larger amounts of faster water than if the water was allowed to percolate into the soil across a catchment. This additional hydraulic stress often damages aquatic habitat by scouring, smothering or eroding sediment as water moves swiftly through the rivulet.

In 2023 the City of Hobart released its first ever State of our Rivulets report. This uses scientific sampling of waterbugs along Hobart's four key rivulets to measure the health of the city's inland waterways.

Waterbugs provide insight into the longer-term effects of water quality on the health of a freshwater ecosystem.

Waterbugs break down and feed on organic matter and each other. They are important food for larger animals in the food chain. This includes native freshwater fish and the much-loved platypus, which inhabit Hobart's rivulets.

The diversity of waterbugs at particular points along a waterway can tell us much about that waterway's long-term exposure to impacts such as pollution and sediment.

The presence of more sensitive waterbugs indicates lesser impacts. Sites populated only by "tolerant" organisms suggest impacts of pollution, sedimentation and erosion. Tolerant waterbugs are those that have resilience to these impacts.

The upper reaches of Hobart's rivulets are excellent reference sites for what healthy local waterways should look like. This is in terms of stream biodiversity and water quality when pollution is low to non-existent.

These upper reaches also act as a "biodiversity bank" by allowing sensitive waterbugs such as stoneflies and mayflies to breed and migrate downstream when conditions allow.

By measuring the ecological health of Hobart's four main rivulets over time through the presence of waterbug diversity, we can determine which areas most need closer monitoring and remediation.

The world of waterbugs is completely alien to most people. But for those willing to explore this miniature aquatic kingdom it is full of mystery, wonder and some of the strangest creatures on the planet.

In Hobart, freshwater macroinvertebrates inhabit the city's streams and rivulets. They are being used to measure the ecolgocial health of our inland waterways.

Mountain shrimp

Mountain shrimp live in the pools of the Hobart Rivulet just above Strickland Falls. They are more common in rivulets higher up the slopes of kunanyi/Mt Wellington.

These animals have barely changed their form since the Carboniferous period, 300 million years ago. They were described from fossils before the first living examples were discovered in Hobart in 1893.

Photo: Copyright © John Gooderham & Edward Tsyrlin

U-sthenid stonefly

As a nymph, these insects terrorise the rest of the waterbug world in Hobart's rivulets. They hunt at dawn and dusk, moving around and under rocks and woody debris, running down sideswimmers and mayflies just as they are turning in for the night.

This waterbug lives as a nymph for a couple of years before emerging as a fully-fledged stonefly to look for a mate. They are quite common along walking tracks of our rivulets between November and June. When startled or threatened the adult stonefly flares up, showing its red hind wing.

Photo: Daniel Rhodes

Cow shrimp

Cow shrimp are the stars of Lambert Rivulet, where for some reason they are more common than in other rivulets. They live in the leaf litter that accumulates behind cobbles in the rivulet.

Cow shrimp are slow-moving, but well armoured, like miniature armadillos.

Photo: Copyright © John Gooderham & Edward Tsyrlin

Sideswimmers

Sideswimmers can grow to the size of a 5 cent piece. Their body is flat, allowing them to slot between cobbles and leaves, which they shred to bits while feeding.

These are the most numerous waterbugs in the upper parts of Hobart Rivulet and are likely an important food source for platypus.

Photo: Copyright © John Gooderham & Edward Tsyrlin

Log cabin caddis

Many caddis are master builders, the larvae create protective structures from materials at hand – sand grains, silk, or in the case of the log cabin caddis, freshly-cut water weed lumber.

The structures caddis build are completely mobile, allowing the caddis to move around underwater in their own mobile mini-home.

Photo: Copyright © John Gooderham & Edward Tsyrlin

Tinsel-gilled mayfly

There are many mayfly species in Hobart's rivulets.

The tinsel-gilled mayfly has distinctively fluffy gills along its abdomen. Absence of mayflies indicates pollution in our waterways, which is why they are the mascot for the National Waterbug Blitz.

Photo: Copyright © John Gooderham & Edward Tsyrlin

Threatened vegetation communities

Eucalyptus globulus dry forest and woodland can be found along upper Lambert Creek, Brushy Creek and New Town Rivulet.

Threatened plants and animals

Many bird species have been spotted along several waterways. Including the grey goshawk, white-bellied sea eagle, masked owl and swift parrot.

Eastern barred bandicoots are found along riparian zones. These can function as a habitat corridor between bush areas and suburban gardens.

Several Galaxias species including the climbing galaxias, spotted galaxias, and the common galaxias. In the Sandy Bay Rivulet, you may notice two 'fish ladders' into the culverts below Parliament Street and Regent Street. These create riffles and pools which help fish travel upstream. They are used more often during breeding season.

Platypus inhabit the Hobart Rivulet, with a few frequently observed in this area. Some sightings have been made as far down as the Royal Hobart Hospital on Collins Street. Others have been found in New Town and Sandy Bay Rivulets.

Native invertebrates, including mayflies, are particularly good indicators of water quality. They are usually only present where the water is free of pollutants.

Many species also use the riparian zone as a corridor between habitat patches, and for food and water. These include endemic species such as the Tasmanian pademelon and the Tasmanian native hen.