With the onset of rainfall after an exceptionally severe fire season, a second phase of danger is beginning in Spain : during heavy rain, ash, sediment and organic debris can flow from burned catchments into streams and rivers in dark runoff. This phenomenon, known in Spain as “ríos negros”, primarily affects waterways, according to the authorities, reservoirs and infrastructure, and can also locally affect water quality.
Different figures for the extent of the fires have been reported, each based on a different date: the Ministry for the Ecological Transition and the Demographic Challenge (MITECO) reported on 18 August 2026 that 254,730 hectares had been affected since the start of the year. On 20 August, media reports citing MITECO put the figure at 262,030 hectares. Higher satellite-based estimates have also been mentioned, but had not been officially confirmed by 23 August. All figures are provisional and comparable only in the context of their respective reporting dates.
“Ríos negros” have already been observed locally: rainfall in areas with extensive burn scars has led to watercourses turning black and deposits forming along riverbanks. Reports from the area around the Picos de Europa described similar effects after early summer thunderstorms. Specialist agencies attribute this to the loss of protective vegetation cover and the resulting increase in surface runoff and erosion.
It is important to distinguish between immediate impacts and medium- to long-term risks. In the short term, sediment loads increase; ash particles and burned biomass can degrade water quality and place strain on infrastructure, from raw water intakes and irrigation systems to turbines and reservoirs. In the medium term, reservoirs may silt up and maintenance costs may rise. The areas most exposed are steep slopes with severe fire damage and little ground cover.
MITECO and several autonomous governments have highlighted immediate measures, such as prioritising erosion-control work on critical slopes, clearing inlets to drinking water and irrigation systems, and more closely monitoring affected catchments. Decisions on the scope and funding of further restoration work are to be based on updated damage maps. Experts are calling for coordinated management across national, regional and local levels to protect particularly vulnerable sites ahead of further storms.
How quickly soils recover depends on slope, soil type and vegetation. Rebuilding stable soil structures takes several years in many areas, but reliable timescales can only be determined for specific sites. One thing is clear: the coming months will determine whether subsequent erosion and silting further exacerbate the damage caused by the fires.
Editor: Andreas M. Brucker (Article created with AI assistance).