**1. Introduction**

Dams across the world have been associated with many negative environmental, social and economic impacts. In particular, dams tend to affect downstream water flow, leading to vegetation species loss. Many dammed rivers around the world have been characterised by decline in species biodiversity, increase in invasive alien plants and pollution. Mumba and Thompson [1] argue that flow disturbance provides stimulus for the establishment of opportunistic plants, particularly alien invasive species. In Limpopo, the most northerly province of South Africa, the impacts of dams have also been accelerated by the alternating wet and dry climatic conditions that greatly affect the flow regimes of rivers. This is because dams create barriers to natural water flow and interfere with ecological processes of riparian zones that influence vegetation composition, richness and diversity [2]. Dams have been associated with water pollution generated from industries and agricultural sectors, such as the release of litter, hot water, pesticides and fertilisers into streams and rivers [3].

district, Limpopo province of South Africa. The major rivers Luvuvhu and Mutshindudi originate from Soutpansberg Mountains and the vegetation types range from co-regions 2.01 Sour Lowveld Bushveld, Soutpansberg Arid Mountain Bushveld and patches of Afromontane Forest to the eco-regions to 2.15 Northeastern Mountain Grassland and Afromontane Forest [8].

Assessment of the Riparian Vegetation Changes Downstream of Selected Dams in Vhembe…

and was raised by means of spillway gates in 1970/1971. The dam was built to supply water to the Levubu Irrigation Scheme to irrigate tropical fruits such as bananas, litchis, mangos, avocados and macadamia nuts (**Figure 1**). Its maximum water carrying capacity is 25.6×106

[6]. It lies 22 km south-east of Makhado (formerly Louis Trichardt) and approximately 45 km west (upstream) of the Nandoni dam, which also lies on the Luvuvhu River. It has a surface

The Damani dam is on the Mbwedi River and was established in 1991 (**Figures 1** and **2**). The

the surrounding commercial farms owned by local community members. Its maximum water

m3

and it has a surface area of 130 ha [6].

07'30″E. It was built on the Luvuvhu River in 1952

http://dx.doi.org/10.5772/intechopen.78329

31'22″E and was built to supply water to the former

per annum. It supplies irrigation water to

m3

67

06'25"S and 30<sup>o</sup>

area of 350 ha. The rainfall station in the vicinity of the dam is Goedehoop.

50'07"S and 30<sup>o</sup>

Damani Coffee Estate which required 4.08×10<sup>6</sup>

**Figure 1.** The locations of selected dams in the study area.

**2.1. The study area**

*2.1.1. Albasini dam*

*2.1.2. Damani dam*

dam is situated at 22o

carrying capacity is 12.4 × 106 m3

The dam is situated at 23o

The decrease in downstream flow is strongly associated with negative impacts on downstream riparian ecosystems. The decrease in water quality and quantity downstream alters the vegetation composition, natural irrigation of floodplains and microclimates. These downstream impacts are associated with a decrease in biological diversity, changes in successional stages, altered biogeochemical cycles and alteration of downstream natural ecosystems. Toxins that accumulate in dams can be released during flush periods and is often accompanied by irreparable damage to the downstream environment. The presence of dam results in landscape modification and changes in hydrology, channel morphology and physiochemical properties result in the dramatic decrease in biodiversity in riparian and aquatic ecosystems (upstream & downstream) [4]. About half of South Africa's rainfall is stored in dams [5]. There are about 550 government dams that carry about approximately 37 million m3 of water. There are about 25 registered dams in Limpopo Province [6].

The dams of Limpopo, with much of the Province characterised by wet-dry climatic cycles have remarkable and sometimes irreversible impacts on downstream ecosystems. These dams usually change vegetation composition, reduce species diversity, alter flow characteristics and encourage alien species invasion that alter the ecosystem functioning of rivers downstream. Need therefore arises that the impact of these dams on their downstream river ecosystems be studied using historical aerial photograph data to determine whether the upstream existence of dams impacts on downstream vegetation density. The main purpose of the study was to use historical aerial photography to assess the impact of selected Luvuvhu/Mutale dams of the Limpopo Province on their downstream river ecosystems. The specific objectives were to study the nature of vegetation changes along downstream river courses, and to compare upstream and downstream vegetation species richness.
