History
Mount St Helens is 40,000 years old, this is relatively young on a geological time scale. It had been dormant since 1857. There have been frequent small eruptions during the last 2,500 years, these have caused pyroclastic flows, ash falls, debris flows, lava domes and lava flows of andesite and basalt. (Simon, 1999)
Mount St Helens, is the fifth highest peak in the Cascade Range and stands at 9,677 feet. At the highest altitudes there is a covering of ice and snow. The mountain base measures 6 miles across. It consists of layers of lava rock and pyroclastic deposits. This type of volcano is usually referred to as stratovolcanoes. (Foxworthy and Hill, 1982)

Hydrologic Setting
The valleys that surround Mount St. Helens were glaciated during the Pleistocene glacial advances. Fluvial and other erosion processes have eroded the valleys to give unique landforms. The average annual precipitation is 140 inches (National Weather Service Data), and about 75% falls between October and March this increases the snow pack on the top of the volcano.
Before the eruption there were dense coniferous forests with clear lakes and streams. These streams were very cold and contained few nutrients or organisms. (Iverson and Martinson, 1986)  

Eruption
On May 18th 1980 Mount St. Helens erupted. A huge debris avalanche preceded the eruption. The eruption caused pyroclastic flows, mudflows and volcanic ash, which covered the United States. (Foxworthy and Hill, 1982)
Two months before the eruption there had been 10,000 earthquakes. The magnitude 5.1 earthquake struck beneath the volcano and started the devastating eruption. (Brantley, 1994)
 
 

River Systems
There are three main river systems that drain the volcano the Toutle River, the Kalama River and the Lewis River. The Lewis River has three dams for hydropower generation. Before the eruption Spirit Lake was impounded in the Toutle River by a natural dam made up of ancient mudflows. These reservoirs were used for recreation. (Keller, 1986)
The Cowlitz River Basin covers an area of 2,480 square miles within this area it includes the Toutle River Basin. The Lewis River Basin has a drainage area of 1,046 square miles. (Dinehart, 1992)

The four main effects of the Mount St Helens eruption on river drainage are described below.

1. Debris Avalanche
At the beginning of the eruption a debris avalanche on a path down the north side of the volcano entered the top 17 miles of the North Fork Toutle River, it deposited 3 billion cubic yards of material. (Water Data Report WA-80-1) This material was deposited in Spirit Lake; it was between 45-180m deep. A new drainage system formed on top of the avalanche as ponds and lakes were breached. Streams followed the initial drainage pattern and eroded narrow channels mainly due to the steep slopes. (Brantley and Topinka, 1984)

2. Mudflows/ Lahars
Mudflows developed after the debris avalanche in the South Fork Toutle River and in the Lewis River tributaries. 11,000 acre-feet of water, mud and debris were deposited in Swift Reservoir in a 3hour period. One of the mudflows originated on the avalanche debris and caused destruction in the Toutle and Cowlitz Rivers. Deposition was so high that the Columbian River had to be closed for shipping. Channel capacity of the Cowlitz River was reduced from 76,000 to 7,300 cubic feet per second. (Water Data Report WA-80-1)
The highest lake on the Lewis River Valley is Swift Reservoir; this receives water from Swift Creek, Pine Creek and Muddy River. Lahars flowed into these streams dumping 18 million cubic yards of sediment into the lake raising the level by 0.85m. Pacific Power and Light who operate the reservoir had been prepared for such an event, and as a result had lowered the water levels so the dam did not break or overflow. (Wolfe and Pierson, 1995)

3. Storm Flows
The eruption stripped the trees from the hillslopes within a radius of 11km of the volcano. This meant that when precipitation took place there was a far higher rate of surface run off, especially when storms occurred. The volcano provided large amounts of loose sediment; this sediment was carried along with surface run off into the rivers. The reduced infiltration rates on the hillslopes and low roughness along river channels, meant streams produced higher peak flows in response to rainfall. The greater stream flows caused an increase in erosion and transportation rates of sediment. Deposition of this debris occurred downstream, reducing channel depth. An example of this is seen in the Toutle River where flooding was caused by the decreased channel depth. (Brantley and Topinka, 1984)
Storm flows eroded the debris avalanche and mudflow deposits, causing the collapse of unprotected bank material. (Dinehart, 1992)

4. Tephra Deposits
The Clearwater River Basin received heavy tephra deposits during the eruption; this increased the sediment load of the river in a similar way to the mudflows, except the result was not as significant. This is because the volume of material deposited was far less than that of mudflows. (Dinehart, 1992)

Features Formed
There were 3 natural lakes formed in the Toutle River by natural debris dams. These would have been unstable and liable to cause flooding should the dams have collapsed, so work was carried out to make them safer. (Wolfe and Pierson, 1995)
These natural dams have caused flood hazards in the area. In August 1980 a dam collapsed draining a 250acre-feet lake in the Toutle River near Elk Rock. The resulting flood caused much damage. (Tilling, Topinka, and Swanson, 1990)

Conclusions
From all the information gathered it could clearly be seen that the debris avalanche, mudflows, tephra deposits and storm flows all had large impacts on the drainage system on and around Mount St Helens. However there is definitely an interaction between all of these factors. Hot pyroclastic material in the debris avalanche along with the tephra deposits melted snow and glacial ice. This freed water then mixed with the sediments to give mudflows. Again the melting of the snow and ice along with rainfall caused the storm flows. Therefore it is hard to separate out the consequences and say which was more important, as one process seems to act as a pre-cursor for another process. However the initial debris avalanche stands out as the most important as it was the trigger of the eruption and it produced greater volumes of sediment than any of the other processes, it also acted as a trigger for mudflows and storm flows.
Since May 1980,there has been a substantial natural recovery of the drainage system around Mount St Helens. Yet during this period of recovery, floods and mudflows damaged many roads, and many homes were lost due to stream-bank erosion.  

Reference List

Brantley and Topinka, 1984, Volcanic Studies at Johnson Cascades Volcano Observatory, Vancouver, Washington.    
        Earthquake Information Bulletin, V.16, n.2 March- April 1984, p111-122.

Dinehart, 1992, Sediment data for the Streams near Mount St Helens, Washington: USGS Open File Report 91-219, p2.

Iverson and Martinson, 1986, Mount St Helens: American Geomorphological Field Group, Field Trip Guidebook and Abstracts.

Keller, S.A.C, 1986, Mount St Helens: Five Years Later. EWU Press.

Tilling, Topinka, and Swanson, 1990, Eruptions of Mount St Helens: Past, Present, and Future.

Water Resources data for Washington, Volume 1, Western Washington, Water Year 1980.

http://vulcan.wr.usgs.gov/volcanoes.html
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Modified from: http://www.brunel.ac.uk/depts/geo/iainsub/studwebpage/tooley/karen.htm