Water Quantity

Upper Missouri Waterkeeper recognizes that flow, or the availability of water quantity, directly imacts water quality within Southwest and West-Central Montana.

We address two specific areas of water quantity, each with the overall goal of ensuring sufficient water remains for streams, rivers or groundwater to sustain healthy, viable ecosystems, healthy fisheries, and good water quality.

Stream dewatering

Looking upriver at the Gallatin from Gallatin Bridge during a warm summer month. Sadly, not much water left here.Fishing rivers in southwestern Montana can be a joyous way to celebrate summer. But, sometimes, summer fishing tells a different story.

Imagine the sickening feeling if you saw a pair of 16-inch brown trout floating belly-up in a trickle of water that was formerly the Jefferson River. Imagine that these dead trout were the remnants of a massive fish kill that occurred because most of the stream’s flow had been diverted for irrigation. Sadly this can and does happen in Montana when minimal snow pack, little spring rain, and a heavy demand for irrigation in spring and summer months reduce a river’s flow in short sections to a trickle at best. When this happens Montana’s beloved trout became stranded in pools that eventually heated up to temperatures that are deadly.

Unfortunately, such stream dewatering is a recurring problem in a number of Western states, and, sadly, the additional water needed to keep the streams flowing is often of more value to other users. Indeed, the truth is that sometimes when fish lay dying in 4 inches of water, six inches of water was is standing in nearby fields.

Upper Missouri Waterkeeper seeks to add value to the good work being performed by local watershed groups and organizations like Trout Unlimited’s Montana Water Project, each of whom help secure valuable instream flows through Montana’s water rights and legal system. Educating and working collaboratively with stakeholders from all walks of life – from irrigators, to ranchers, to recreationalists – has proven the most effective means to protect waterways and fisheries from harmful dewatering.

Groundwater dewatering

Groundwater dewatering in Montana is an activity that typically requires a permit. Upper Missouri Watekeeper watch-dogs applications for dewatering to ensure that best available science and consideration of nearby water resources, fisheries and wetlands are considered and protected in the permitting process.

There are a wide range of potential impacts from dewatering and groundwater lowering. These can be categorized in various ways, such as the following groups of impact types:

Groundwater dewatering by pumpingGeotechnical impacts

Dewatering can cause ground settlements which, in some cases, can be large enough to cause distress or damage to any structures located within the zone of drawdown. Ground settlement is an unavoidable consequence of all cases where groundwater levels are significantly lowered, but in the majority of cases the resulting settlements are too small to be damaging.

Settlement can also occur where dewatering is poorly executed without adequate filtration. There fine particles may be drawn out of the soil in the pumped water; this will loosen the soil, and in extreme cases, can create underground voids which can collapse unpredictably and cause localized large settlements.

Contamination impacts

When pumped by wells or sumps hydraulic gradients are generated drawing groundwater towards the well. If dewatering is carried out on or near a site which has legacy groundwater pollution, these hydraulic gradients can cause existing contamination to move and migrate toward the dewatering system. If the contaminated area is very close to the dewatering system then contaminated water may emerge in the pumped water, requiring water treatment before it can be discharged.

Water dependent feature impacts

Natural groundwater flow plays an important role in sustaining many natural water-dependent features such as rivers, springs and wetlands. As well as their obvious function in transmitting or storing water, many of these features also form important habitats and ecosystems.

Groundwater control by pumping can potentially intercept groundwater which would normally reach these water-dependent features. If this occurs then river and spring flows may reduce or water levels fall in wetlands and ponds. There is also the potential for groundwater control by exclusion to affect water-dependent features. If deep and extensive cut-off walls are installed as part of groundwater exclusion schemes may block or divert natural groundwater flow and potentially can have a negative impact on any nearby features.

Water resource impacts

Where dewatering pumping is carried out in an aquifer which is used as a source for water supply by third parties (for example for drinking water or for industrial water use) there can be negative impacts on the available water resources from the aquifer. These types of impacts are most commonly of concern where a dewatering system is to pump high flow rates for extended periods of time (several months or longer).  In that case the sustained dewatering pumping may have the potential to lower regional groundwater levels in the aquifer, and reduce the water resources available to third party abstractors. This may be apparent in the short term in the form of lowered water levels in water supply wells and correspondingly reduced yields.