Sometimes hydrologic systems have been altered with unintended consequences. Agriculture and city drains have sent water to oceans without recharging the groundwater, thus allowing salt water intrusion into coastal aquifers. Large dams often block fish migrations, although fish ladders offer partial solutions; large dams also catch sediment and release sediment-free water which can scour and lower downstream alluvial floodplains. River diversions and irrigation systems sometimes take fish into irrigation ditches unless appropriate guards are installed. Watersheds not managed to protect erosion can cause a reservoir to fill with silt. All above consequences can be partially mitigated, although tradeoffs are incurred. Water shortages are mitigated by conserving water through efficient use; by recycling and treating/reusing water; and by obtaining more water sustainably. Rainwater rooftop harvesting is common in some places; artificial glaciers and tapping glacial lakes can provide water in mountain regions; and desalinization is becoming efficient. Water is occasionally shipped in pipelines. If fossil fuels become less used, some oil pipeline may be used to ship water instead.