Across large parts of northern Tanzania, gully erosion - soil erosion caused by flowing water - is cutting deep scars through fertile farmland, grazing areas, roads and even villages. These gullies grow faster every year and what was once a slow environmental process has accelerated into a humanitarian threat. It has serious consequences for food and livelihood security, infrastructure and biodiversity.
Soil erosion is a natural process. Rainfall breaks soil into particles, and flowing water transports them downslope into rivers and lakes. In Tanzania, however, erosion has intensified dramatically over the past 120 years.
The region's steep terrain, highly variable rainfall and fragile volcanic soils make it naturally vulnerable. What has turned this into a crisis is the change in how people interact with the land.
The Jali Ardhi (Swahili for "Care for the Land") programme is an international collaboration of scientists from several universities. We use interdisciplinary tools to investigate what's causing increased soil erosion and how communities can restore the land. Results from multiple projects over a decade point to runaway gully erosion as a key driver of land degradation in Tanzania. Urgent and widespread restoration programmes are needed.
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From fertile volcanic soils to runaway gully erosion
The Tanzanian highlands are blanketed by soils formed from volcanic basalt erupted over millions of years as the East African Rift opened. These volcanic soils are rich in fine clay minerals with high levels of exchangeable sodium and calcium. Contact with water after a dry period can cause them to rapidly disperse into fine particles in the water.
Normally, these soils are covered by a stable topsoil layer protected by plant cover, roots and soil organic carbon. But land clearing - the removal of natural vegetation for agricultural purposes - and overgrazing remove the natural protection of these soils and increase runoff. When intense rainfall follows, water rapidly flows downhill, concentrating in valleys and carving out gullies.
Indigenous land conservation practices such as seasonal grazing and shifting cultivation recognised this vulnerability by allowing the vegetation and soil to recover. However, these were gradually eroded by colonial and postcolonial governance, which prioritised formal land tenure and permanent settlement but paid little attention to soil productivity and protection from erosion. In the meantime, Tanzania's population has been growing fast, doubling roughly every 25 years for the past century, and is now exceeding 70 million.
Large areas of natural forests and savannahs have been cleared for agriculture. Pastoralist groups, such as the Maasai, were forced into permanent settlements, replacing their traditional practice of moving livestock seasonally to follow rainfall and fresh pasture. Livestock densities have tripled over the past half century and areas that were once allowed to recover are now farmed and grazed year-round, leaving soils permanently exposed to the elements.
Rainfall in these regions is naturally erratic, alternating between dry spells and wet years. We have not observed a long-term change in rainfall but wet conditions following drought can trigger massive erosion events.
Together, these pressures pushed the landscape past a critical threshold. Once gullies form in these volcanic soils, they are difficult to stop. Like a boulder pushed downhill, erosion accelerates once it starts. Gullies can continue growing even if the original trigger, such as deforestation or overgrazing, has ended. They form highly connected channels that quickly remove rainfall, nutrient-rich topsoil, and seeds away from the land. This makes it difficult for vegetation to recover and the landscape to stabilise.
The cost of losing land
We calculated that the erosion rates in Tanzania are about 20 times higher than they were a century ago. Over 50% of the total area of Tanzania is experiencing rapid land degradation. It is one of the fastest degrading areas in the world.
When land is lost to erosion, so too is food and income. Since roughly 70% of Tanzanians are smallholder farmers and they produce most of the country's food intake, over 50% of the population has already experienced moderate to severe food insecurity. The consequences are potentially disastrous.
To make matters worse, we saw from repeat visits and photographs that these mega gullies undercut roads and bridges as quickly as a decade after construction. This is an enormous loss in a country working to develop basic services. Farmers tell us that they are cut off from market access and are forced to grow less-perishable or subsistence crops such as maize and beans, instead of higher-value agricultural production. This reinforces low-income and low-investment cycles.
The effects do not stop here, though.
Eroded sediments fill reservoirs and lakes, threatening water availability, fisheries, biodiversity and tourism. We found that Lake Manyara National Park, a Unesco Man and Biosphere Reserve, is filling up due to the enormous amounts of sediment coming from its catchment. This places pressure on an ecosystem that supports more than 350 bird species as well as iconic wildlife such as elephants and lions.
Working together to join the cracks
Despite the scale of the problem, solutions do exist. Across east Africa, communities have long used indigenous techniques such as earth bunds (banks), stepwise terraces, leaky dams, and re-establishment of grasses and trees. These work with natural processes and materials to slow water flow and capture soil.
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NGOs such as the LEAD foundation and Justdiggit are revitalising these community-led approaches. We set up a soil and gully monitoring network combining scientific sampling and sensor technology with citizen science approaches to evaluate the evolution of gullies and success of restoration approaches.
In some cases, collective community action has successfully stabilised small gullies and improved soil quality. But many mega gullies are now too large for resource-poor communities to address alone. While the Tanzanian government has committed to the United Nations Convention to Combat Desertification's Land Degradation Neutrality initiative, long-term investment and action plans are lacking. This is partly due to the mismatch between national politics and local action needs.
Read more: Key insights into land degradation from seven African countries
Ultimately, erosion control cannot be treated as a solely environmental problem. Halting this crisis will require coordinated and large-scale land restoration investment, while simultaneously addressing socio-economic issues linked to poverty, corruption, education and economic development. More than half of Tanzanians are under 18, growing up on landscapes already under strain. Whether these systems can continue to support a growing population will require action before the cracks in the earth widen any further.
Maarten Wynants, Marie Curie Global Postdoctoral Fellow, Ghent University
