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MARTHA  BRAE  WATERSHED

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M Brae Watershed map

Rivers in the Martha Brae Watershed

The only rivers in the Martha Brae Watershed that carry substantial year-round flow are the Martha Brae / Roaring River in the north and Mouth River to the south.

Martha Brae river rises at Windsor and joins with Roaring River at Unity.
The source of Roaring River lies in the Maldon Inlier, which is drained by the Tangle River which sinks and then reappears at Spring Vale Cave as the Springfield River. It flows NE across the polje to sink again and reemerge at the Deeside Rise as the Roaring River. The combined rivers flow northwards then eastwards near Good Hope.
About one mile after the Good Hope bridge, the river turns northwards to the Potosi Waterboil which is a group of boiling springs at an elevation of about 25m and discharging at about 1,700 l/sec.
North of the Waterboil the river flows through a narrow gorge which opens again south of Southfield (Rafters Village).
Main Gully, which only carries water after heavy rains, joins the river here.
Near Irving Tower the river emerges into the coastal plain and flows across its own alluvium-covered flood plain to enter the sea at Oyster Bay, between Falmouth and Rock.

The characteristics of the Martha Brae surface hydrology can be summarised as follows.

  • Many of the upper tributaries have infrequent surface flow and possess no definite stream channel.
  • Drainage density is not remarkable when compared on a global scale.
  • Streams in limestone valleys have irregular hydrologic regimes. Some streams have permanent water flow while others alternate between wet and dry reaches. This indicates that the pattern of valley drainage systems is attributed to surface run-off under conditions of high rainfall.
  • Under non-flood conditions, many of the valley systems are virtually inactive in that they carry very little or no surface flow. The base flow is supplied by underground drainage systems.

The Martha Brae River at the Martha Brae and Pantrepant stations has a sustained stream flow in the dry season. However, at the Friendship and Bunkers Hill stations, both the Martha Brae and Roaring Rivers have a much less sustained flow. The water derived from adjacent drainage sinks is insufficient to maintain the base flow volumes.

Dry Martha Brae near Retreat
Water flow under flood conditions within the valleys is a function of two factors, surface run-off generated under rainfall conditions and ground water derived from subterranean drainage systems. The volume of base flow is an important component of the total volume of flood run-off. Therefore, the typical flood hydrographs for the watershed are characterised by a gradual flood rise and long duration of base flow after the end of storm events.
In steeply sloped valleys which experience periods of heavy rains, flow debris is very common in the upper parts of the valleys. Extensive fan-shaped deposits of coarse debris, composed of limestone gravel and cobbles, can be seen in the lower reaches of these valleys. Where stream networks are a function of perennial karstic base flows, valley stream systems are the products of ephemeral storm flow. The longitudinal profiles within the valleys are highly irregular over short distances. In part this is due to outcrops of particularly resistant limestone beds. A sequence of drops formed by these limestone outcrops can be seen downstream of the Martha Brae rafters’ station.

The Mouth River drains the northern edge of the Central Inlier and sinks near Spring Garden to resurge 11 km to the north at the Fontabelle Rising.
Fontabelle Rising is also fed by Printed Circuit Cave and runs a few hundred meters on the surface to sink again and rise as the Potosi Springs within the bed of Martha Brae. A small portion of this flow also rises at Bunkers Hill

Ponds are found in the perched watertable which corresponds to the geological transition zone along the Duanvale fault. The low permeability of the clay and silts which mantle the floors of the basins in the transition zone cause the retention of a large amount of the rainfall which falls on the clay and which is held in the clay itself or in the form of small superficial ponds. In this way a secondary “perched” watertable may be formed above the underlying White Limestone.
This perched watertable does not seem to bear any relationship to the water table level in the limestone below.