Northwestern Glaciated Plains Intermittent Stream
Provisional State Rank
(see reason below)
State Rank Reason
The number of quality occurrences in the state is unknown, but probably fairly common. This is a difficult community type to quantify given the past years of drought in the state and the tenuous nature of this aquatic system. Intermittent pools containing the Ostracoda group should be inventoried for unique crustaceans, such as fairy or tadpole shrimp.
This ecosystem is found widely throughout the coulees, small streams (1st to 3rd order, average wetted width=3m) and headwaters of Medium Prairie Rivers (B006) and Northern Glaciated Prairie Streams (C006, C008) within the Northern Glaciated Plains Ecoregion. These small, cool to warm-water, low gradient and elevation (900-1200m) streams have origins in the alluvium and sedimentary geology of this region. Throughout their range these clear (typical) to turbid streams are usually characterized by vegetated long pools separated from each other by narrowed riffle areas, which are usually dry by early summer, forming isolated pools or a “string of pearls.” Once these systems lose their connectivity to the fish recruitment pools of downstream reaches (sometimes due to climatic factors such as drought), or if they never had fish potential, they become the Fishless Isolated Pool ecosystem type (E006). The substrate can be gravel to cobbles in the shallow pools, to silted gravel deeper pools with rooted vegetation. These pools, on average, remain filled longer and are cooler and wider than their Great Plains counterparts. The D006c is a prairie stream reach within 1 river mile of a confluence with a larger river (at least 2 stream orders larger) and contains a biological fish community that is significantly affected by the large river connectivity.
Intermittent stream indicator species rely on the seasonal to permanent channel pools as essential habitat. These resident and pioneering fish species, sometimes also referred to as obligate species, are dependent upon these unique pools for their continued existence. One of the defining characteristics of the seasonal pool biotic community is a lack of permanent populations of predatory fish. In the largest clear, non-degraded pools, members of the Northern Redbelly Dace Assemblage may occur. A reference condition Northern Glaciated Intermittent Stream type will have fathead minnow, brook stickleback, lake chub (not as common), brassy minnow and northern redbelly dace in order of dominance. If connectivity to downstream reaches exists on an annual basis, the pioneering white sucker may occur. Without aquatic macrophytes in the pools, brook stickleback or northern redbelly dace will be rare. Typically this community will be co-dominated by fathead minnow and brook stickleback only, and just fathead minnow in truly degraded or non-vegetated systems. The E006 type will be fishless. If there is aquatic vegetation in the pools, brook stickleback may be present, but often it is just a single fish species in the pool. The only introduced fish species reported from a D005 stream was black bullhead. Isolation and periodic drying keep predatory fish from colonizing and occupying the string of pools. The macroinvertebrate community consists of members of the Prairie Stream and Pool Assemblages, occurring in the cobbles and in the vegetated pool areas respectively. The reference community indicator species are characterized by the crustaceans, Hyalella and Gammarus; damselfly genera, Coenagrion/Enallagma sp. Enallagma civile, Ishnura; many genera and species of the water boatman (Corixidae), Sigara alternate, Sigara grosslineata, Trichocorixa, Trichocorixa nais, and Corisella; the snails, Physella, Gyraulus, and Stagnicola; the mayflies, Caenis and Callibaetis; and beetles Haliplus, Oreodytes, Laccophilus, Hydroporus and Hygrotus. As the complexity of the pool habitat decreases, the system loses clinger habitat species such as the damselflies and many of the water boatman taxa. The truly intermittent fishless pool ecosystems (E005) may only exist for a few months. If these pools are dry for more than a year and then rehydrate, many invertebrates with resting egg stages dominate the pools, including the crustaceans Ostracoda, Cladocera, Copepoda; the fairy shrimp, Branchinecta, Eubranchipus; the clam shrimp, Caenestheriella; and the tadpole shrimp, Lepidurus.
The Northern Glaciated Intermittent Stream Ecosystem occurs throughout the glaciated Great Plains regions of northern North America. Within Montana, this community exists in a multitude of streams and coulees mostly situated north of the Missouri River, including Murray Coulee, Cowen Coulee, Hell Coulee, and Rattlesnake, Little Sage, Coberg, E. Fork Battle, Black, Little Sandy, Snow, Bitter, Buggy, Plentywood, Boxelder and E. Fork Porcupine Creeks.
Intermittent stream pool indicator species rely on seasonal pools as essential habitat. These species, sometimes also referred to as obligate species, are dependent upon these unique pools for their continued existence. One of the defining characteristics of the seasonal pool biotic community is a lack of permanent populations of predatory fish. Isolation and periodic drying keep predatory fish from colonizing and occupying the pool.
Small dams, water diversions, stock ponds and introduced gamefish species have had the most significant negative impact on this community (Winston et al. 1991). Other threats include heavy cattle intrusions to the riparian areas, which causes bank erosion and subsequent sedimentation and siltation. Anywhere dams occur, even small stock pond earthen dams, the downstream reaches are affected by altered water temperatures, unnatural water level fluctuations, and changes in sediment and nutrient transport. The spring flows backed up behind stock pond dams could have potentially filled numerous E005 pools, and possibly connected these pools for a brief period to downstream fish populations for recolonization. Therefore, unless there is sufficient outflow, streams and coulees below stock ponds usually do not develop this community type. Another threat to this community includes cattle intrusions, which cause trampling of stream banks with the subsequent siltation and nutrient inputs (cow pies) into the pools. This siltation and nutrient loading may eliminate aquatic macrophytes and cause blue-green algae blooms. This native community, as well as others downstream, suffers from cattle damage to the riparian areas of these pools. Therefore, riparian areas adjacent to streams should be given priority management attention.