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The lake is situated (34^o26'S; 20^o 22'E) on the coastal plain between 4m and 11m above the mean sea level.  It is 18 km long and 0,5 km wide with a surface area of 6.2 Km² (750ha) when full. The depth of the lake is very variable from a maximum of 7 m during periods of flooding (only twice this century) to nearly completely dry (at least once this century) (Ref.2), causing the salinity to drop from 60ppt to 3ppt within a period of only 2 months (Ref.3). During occasions of extensive flooding, which has occurred only twice this century in 1906 and 1957 (Ref. 2), an area of up to 3000 ha on the plain southwest of De Hoop lagoon may be inundated to a depth of up to 3 m.

The closest town, Bredasdorp is 65 km from the Ramsar site.  The site is within the De Hoop Nature Reserve, which lies along the southern coast of the Western Cape Province .

The Sout River and its tributary the Potteberg River are the most important rivers feeding the De Hoop Vlei. There are also several fountains that discharge water into the northern part of the lagoon, which explains why this area never becomes as brackish as the rest of the lagoon.

The statistics (Ref. 1) for the Sout River catchment is as follows:

• Total surface area = 127 000 ha

• Mean annual rainfall = 371 mm

• Mean annual evaporation (MAE) = 1430 to 1470 mm

• Mean annual runoff (MAR) to De Hoop Vlei = 17 mm or 21,9 million m³ (4,6% of rainfall)

• Area under irrigation = 600 ha

Three wetland types are identified at De Hoop Vlei, namely:

• Coastal freshwater lagoon

• Coastal brackish lagoon

• Seasonal freshwater marshes

The Ramsar site is part of the De Hoop Provincial Nature Reserve, owned by the state and managed by the Provincial Conservation Authorities. The terrestrial portion of the reserve covers an area of about 35 546 ha (355 km²) while the marine portion of the reserve covers an area of approximately 25 300 ha (253 km²).

The provision of environmental education programmes receives high priority. An environmental education centre with accommodation for 60 persons has already been built on the eastern section of the reserve. Environmental education courses have been offered for groups of school children from 1985. At De Hoop an interpretative centre has been developed and walks and hikes will be offered in the near future.

There are proposals in the management plan to develop bird hides and a self-interpretative nature trail along the edge of the wetland. The possibility of zoning the wetland to cater for a limited amount of canoeing is also being looked at.

The boundary of the Ramsar site has been specified as the edge of the lagoon. Therefore the only broad habitat type occurring within the site is the aquatic habitat.

According to (Ref. 6) the vegetation of the surrounding area is Coastal Macchia on limestone substrate. Low and Rebelo (Ref. 7) classify this vegetation type as Limestone Fynbos.

According to the new vegetation classification for South Africa (Ref. 8) three major vegetation types are recognized in the surrounding reserve, viz. Limestone Fynbos, Mountain Fynbos, Dune Fynbos and Dune Thicket. Only very small areas of Laterite Fynbos, South and Southwest Coast Renosterveld and Afromontane (Knysna) Forest also occur. The greater part of area that is mapped as Dune Thicket in this area by Low and Rebelo (Ref. 8) is actually Dune Fynbos with smaller interspersed patches of Dune Thicket.  Melkhout thicket can also be regarded as a form of Dune Thicket. Sideroxylon inerme (White milkwood) trees are common along the lake.

The waterplant, Potamogeton pectinatus, contributes largely to the maintenance of the rich animal life in the vlei habitat and provides favourable habitats for other aquatic organisms such as zooplankton (Ref. 9 and 10) and abundant fish populations (Ref. 11).

Invertebrates:   The waterplant, Potamogeton pectinatus, contributes largely to the maintenance of the rich animal life in the vlei habitat and provides favourable habitats for other aquatic organisms such as zooplankton (Ref. 9 and 10)

Birds:  To date 259 bird species, which represents 70% of the 369 species known in the south-western Cape , have been recorded in the De Hoop Nature Reserve (Ref. 8).   This high species richness is ascribed to the high habitat diversity in this area (Ref. 15).

At least 75 bird species, which are dependent on wetlands, have been recorded at De Hoop Vlei, including 12 of the 18 South African waterfowl species (Ref. 8). Regular monthly or quarterly counts of the birds on De Hoop Vlei have been undertaken since 1979 (Ref. 8).

Noteworthy numbers of birds recorded were recorded of the Yellow-billed Duck (Anas undulata) (Ref. 16), the Cape Shoveller (Anas smithii) (Ref. 17), the Redknobbed Coot (Fulica cristata) and the Egyptian Goose (Alopochen aegyptiacus).

Rare and threatened species that have been recorded are the White pelican (Pelecanus onocrotalus) (Ref. 19), the Little Bittern (Ixobrychus minutus) (Ref. 19), the Black Stork (Ciconia nigra) (Ref19), the Greater Flamingo (Phoenicopterus ruber) (Ref. 4), the Lesser Flamingo (Phoeniconaias minor), the Caspian Tern (Hydroprogne caspia) (Ref. 8) and the Chestnutbanded Sandplover (Charadrius pallidus) (Ref. 8).

De Hoop Vlei is thus an important foraging area for the White Pelican, which is regarded as rare in South Africa with only two breeding populations, namely on Dassen Island and Lake St. Lucia (Ref 17).

A notable event was the first reported large scale breeding attempt of the greater flamingo Phoenicopterus ruber in South Africa in this area in 1980 when up to 5000 birds were present and approximately 800 nests were built with a fledgling success of 35% (Ref. 4).

Mammals:  

As the boundaries of the De Hoop Ramsar site is restricted to the high water mark of the coastal lake a description of the geology of the site would be pointless. The following is an account of the geology of the surrounding area, the De Hoop Nature Reserve.

The northern boundary of the reserve is characterized by the high-lying terrain of the Potberg range and the Hard Dunes (limestone hills) with a maximum height of 611 m and 224 m above sea level respectively. A hard calcrete capping less than 0,5 m thick generally occurs in the older limestone terrain with softer material below.

The Potberg represents the remnant of a syncline of the Cape Folded Ranges which is truncated to the south by a major fault at the base of the range. The Table Mountain Group (TMG) quartzite strata dip northwards forming steeper slopes and cliffs on the southern side (scarp slopes) and more gently dipping northerly slopes (dip slopes). Thick boulder talus and alluvial fans occur at the southern foot of the range. The TMG quartzites form sea cliffs where they are exposed beneath the Bredasdorp limestone.

The basement geology of the area comprises sedimentary rocks of the Table Mountain Group (quartzites), Bokkeveld Group (shales and mudstones) and Uitenhage Group (mainly shale conglomerates). The resistant quartzites of the TMG form the Potberg range while the softer shales and conglomerates have been planed by marine transgressions into a gently southward sloping series of terraces. To the north of the Reserve the Bokkeveld shales have been eroded into undulating hills. Remnants of the Tertiary (25 million year BP) "African I" erosion surface (peneplain) survive as isolated flat-topped hills capped by silcretes of the Grahamstown Formation.

The greater part of the reserve is underlain by Tertiary limestone of the Bredasdorp Formation. These limestones cover most of the Bokkeveld and Uitenhage basement rocks within the reserve. Exposed Bokkeveld Shales and only very small exposures of the red conglomerates of the Uitenhage Formation can be found in the eastern section of the reserve. The Bredasdorp limestones were deposited as shallow marine environments (the De Hoop lagoon formation with shell fossils) and as coastal dunes (most of the rest of the deposits). The oldest deposits (Wankoe Formation) form the higher lying Dunes into which the coastal plain had been eroded during subsequent marine transgressions. Subsequently new dune systems (Klein Brak and Waenhuiskrans Formations) were formed on the coastal plain. The most recent member of the Bredasdorp Group was deposited within the last 10 000 years as a strip of unconsolidated dunes along the coast (Strandveld Formation).

The sandstone and quartzite are approximately 4000 million years old and the limestone 5 million years.

The site lies within a Winter Rainfall area. The lagoon is therefore at it's fullest during or just after winter. The water at this stage is relatively fresh, but gradually becomes brackish as it evaporates during the summer months.

Precipitation:  

The mean annual rainfall is approximately 380 mm (De Hoop Nature Reserve Office) with the maximum mean monthly rainfall occurring in August and the minimum in December and January. Rainfall can however vary by 15 % - 17 % from one year to the next. Summer rains commonly occur as cloudbursts, but rainfall is predominantly cyclonic associated with eastward movement of low-pressure cells crossing the South-western and Southern Cape (Ref. 14 and Ref. 21. Orographic rainfall may account for large differences in rainfall between the lowlands and the high-lying ground such as the limestone hills and the Potberg, particularly towards the eastern extremity of the Potberg. Rainfall on the Dunes may exceed 400 mm and that on the Potberg may exceed 700 mm per year (1:250 000 isohyet value). The mean annual precipitation for the Sout River catchment is 369 mm (Ref. 21).

The wettest 3 months of the year is June, July & August (12% of MAP per month) while the driest 3 months of the year is December, January & February (4 to 5,5% of MAP per month). Rainfall is fairly evenly distributed over the year (7 to 12,5% of MAP per month) except for the 3 driest months (4 to 5,5% of MAP per month).

Precipitation in the form of mist occurs in autumn and winter. At times the whole Overberg area can be covered in a thick mist bank. 

Frost and hail occur occasionally

Temperature:  

The warm Agulhas current results in temperate winters and warm summers. Temperature averages 16,8C per annum with an average of 20,5C and average winter minimum of 13,2C (Ref. 21).  The warmest month is January with a mean air temperature of 22C. The coldest month is July with a mean air temperature of 11C.

Wind:

Windy conditions are common particularly in summer when the prevailing wind direction is south-westerly with an average velocity of 35 km/h. Wind speeds may reach 60km/h or more at times.

The development and operation of the Overberg Test Range has not resulted in a significant disturbance to the bird life of De Hoop Vlei as the aircraft flights are kept well away from the lake. The possible impact of the missile testing range is being monitored and strict requirements have been laid down for the operation of the testing range to ensure that disturbance is kept to acceptable minimum levels.

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