# What influences erosion? <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Geology </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> About 68 per cent of the Horizons Region’s 2.2 million hectares is steep, rolling hill country and contains 17 per cent of Aotearoa New Zealand’s highly erodible land. Much of this land is underlain by mud, silt, or sandstone - rock types that are naturally prone to slipping, slumping, and other forms of erosion. </div> </details> <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Land-use activities </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> Erosion is a naturally occurring process, but human activities can accelerate it. The region has the largest area of hill country farming in New Zealand, with over 260,000 hectares of highly erodible land in pasture. The impacts of the hill country’s fragile geology are more pronounced in these grassy expanses of farmland, as there are fewer trees and other woody plants to stabilise the soil. </div> </details> <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Severe weather events </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> Intense rainfall during storm events often triggers landslides on steep terrain. The February 2004 storm highlighted the region’s vulnerability to severe weather events. About 70 per cent of the Horizons Region was affected by the weather, with 62,000 individual landslides recorded, and an estimated 200 million tonnes of soil lost across the region. During the event, 28 tonnes of <a href="https://www.lawa.org.nz/learn/glossary/s/sediment">sediment</a> passed under Palmerston North’s Fitzherbert Bridge every second for eight hours. </div> </details> <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Climate change </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> Climate change is likely to exacerbate erosion in the Horizons Region, as rainfall plays a key role in washing away erodible soils from the land. Climate change projections by NIWA for the Ministry for the Environment predict that, despite expecting less rain annually by the end of the century, we will likely experience heavier rainfall events when it does rain. The ministry’s projections also indicate that most of the region will experience more frequent heavy rainfall days by the end of the century. </div> </details>

# Why do we care about erosion? <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Environmental impacts </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> When eroded soils enter waterways, they become part of the sediment load transported or deposited throughout rivers, lakes, estuaries, and the sea. Here, sediment can adversely affect aquatic life, as its fine particles can fill the small spaces between rocks on the riverbed, making the habitat unsuitable for fish and aquatic insects. Sediment can also smother the riverbed (reducing oxygen transfer and creating anoxic areas) and introduce additional loads of phosphorus bound to sediment (which promotes excessive algal or plant growth). These particles can also make the water cloudy, obscuring people’s view of hazards in waterways and making the water unpalatable for stock. </div> </details> <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Economic impacts </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> In the Horizons’ Region, storm-induced landslides have damaged farm infrastructure, disconnected roads, tracks and raceways, and caused landowners to lose livestock and crops. The 2004 storm event caused about $240 million in agricultural losses in the region. More recently, Cyclone Gabrielle (2023) caused at least $7.8 million in damage to farms in the Tararua District. Landslides also lead to the loss of topsoil and its productive capacity. Topsoil provides essential support for plants to grow but when this vital resource is lost, it can take hundreds of years to develop again. </div> </details> <details style=" border: 1px solid #d0d7de; border-radius: 14px; padding: 0.9rem 1.1rem; margin: 1.2rem 0; background: linear-gradient(to bottom, #ffffff, #f8fafc); box-shadow: 0 3px 10px rgba(0,0,0,0.05); "> <summary style=" cursor: pointer; font-weight: 700; font-size: 1.08rem; color: #1f2937; outline: none; "> Flood management impacts </summary> <div style=" margin-top: 1rem; line-height: 1.7; color: #374151; font-size: 0.98rem; "> Sedimentation from accelerated erosion can reduce the river channel’s capacity to transport floodwaters. Severe weather events transport eroded soils and associated debris throughout Horizons’ river and drainage schemes, impacting flood protection assets and causing sediment to accumulate in rivers, raising the riverbed and reducing the river channel's capacity to transport flood flows. <br> Sedimentation is especially problematic along the Ōroua River, as its berms can be more than a metre higher than the land directly outside the stop bank. This reduces the river’s carrying capacity during high-flow events. </div> </details>