NASA put up a nice before and after image (see above) of a landslide along a glacial margin in Alaska earthobservatory.nasa.gov. This slide was caught on seismic monitors and the approach of identifying large slides has been followed by Dave Petley at the landslideblog.
A similar landslide event took place this spring along a very steep slope on the west side of Mount Baker mbvrc/update-nooksack-debris-flow-initiated-by-landslide-not-outburst-flood/.
Essentially the glacier erodes the toe of the steep mountain slopes, but as long as the ice remains the steep slope is buttressed by the ice. When the glacial ice retreats the steep slopes no longer have the mass of ice buttressing the slope and will be subject to failures. The sediment slug added to the stream or river then has to be 'processed' through the river system. The big stuff takes some time.
During the late May 2013 event on Mount Baker turbidity gauges along the river recorded very high readings as the fine sediment passed downstream. The silt and clay ultimately gets deposited in Nooksack River delta and Bellingham Bay.
A very muddy Nooksack River in early June 2013
Many of the larger glaciers that extend down river valleys off of the Cascade Range volcanoes have retreated significantly the past several decades leaving lots of unsupported steep slopes. Hence, sediment loads to river systems fed by these streams and rivers may be receiving a significant increase in episodic slugs of sediment.