How do human activities change river landscapes, which alter storm hydrographs?

Human activities such as urbanisation, land-use change, and deforestation are reshaping river landscapes and altering how rivers respond to rainfall. These changes directly affect the storm hydrograph, which shows how discharge in a river rises and falls after a storm event.

Urbanisation

Urbanisation increases the amount of impermeable surfaces (roads, pavements, rooftops), which significantly alters the natural drainage of an area.

Effects on River Landscapes:

• Reduces natural infiltration and groundwater storage
• Increases surface runoff
• Rivers are often straightened or culverted (put in concrete channels)

Effects on Storm Hydrographs:

• Shortens lag time (faster runoff into rivers)
• Increases peak discharge (more water reaches rivers quickly)
• Steeper rising and falling limbs, creating a “flashier” hydrograph

Example: Urban expansion in cities like Manchester and Birmingham has led to increased flood risk due to impermeable land cover and storm drains feeding water rapidly into rivers like the River Irwell.

Land-Use Change

Land-use change includes turning grasslands or wetlands into farmland or built-up areas, often disrupting the natural water cycle.

Effects on River Landscapes:

• Replaces vegetation that previously slowed down water
• Alters sediment loads and can lead to channelisation
• May involve drainage systems that bypass natural infiltration

Effects on Storm Hydrographs:

• Less water is stored in the ground
• Runoff increases, especially during heavy rain
• Hydrographs show shorter lag time and higher peak discharge

Deforestation

Deforestation removes trees and vegetation that help intercept rainfall and slow the movement of water into rivers.

Effects on River Landscapes:

• Exposes soil to erosion, increasing sediment in rivers
• Reduces evapotranspiration, increasing total runoff
• Makes slopes unstable, increasing mass movement

Effects on Storm Hydrographs:

• Less interception, so rainfall reaches the ground quickly
• Greater overland flow and surface runoff
• Flashier hydrograph with reduced lag time and sharp peak discharge

Example: Deforestation in upland areas of the River Severn basin has been linked to faster river responses and higher flood risk during storms.

Human activities are altering the natural balance of river systems. The replacement of vegetation with hard surfaces, drainage networks, and deforestation all contribute to storm hydrographs becoming more “flashy” with:

• Shorter lag times
• Higher peak discharges
• Increased flood risk

Sustainable urban planning and land-use management are crucial for mitigating the impacts on river landscapes.